Plunger kit, prefilled syringe, and method for manufacturing prefilled syringe

Abstract

[Problem] To provide a plunger kit capable of reducing a risk that a drug in a syringe barrel is exposed to a normal environment when a plunger rod is attached to a gasket. [Solution] A plunger rod 30 includes: a rod body 31 extending in an axial direction; and an attachment portion 33 provided at a distal end of the rod body and attachable to a gasket 20. The attachment portion includes an engagement portion 33b provided on a distal end side of a shaft portion 33a and engageable with an accommodation portion 24c.

Description

Plunger Kit, Prefilled Syringe, and Method for Producing Prefilled Syringe

[0001] The present invention relates to a plunger kit, a prefilled syringe including the plunger kit, and a method for producing a prefilled syringe.

[0002] A prefilled syringe used for injecting a medical fluid includes: a syringe body (syringe barrel) provided with a medical fluid storage chamber; a medical fluid filled in the medical fluid storage chamber; a cap attached to a nozzle of the syringe body; a piston (gasket) slidably fitted inside the syringe body; and a plunger rod for sliding the piston in the axial direction. In such a prefilled syringe, a syringe body with a cap attached to a nozzle is filled with a medical fluid, only the piston is stoppered first to seal the medical fluid in the syringe body, and the distal end portion of the plunger rod is pushed into a recess (opening) on the proximal end side of the stoppered piston to join the plunger rod and the piston, thereby assembling the prefilled syringe.

[0003] Incidentally, when connecting a plunger rod to a piston, it is necessary to push the plunger rod in such a manner that the plunger rod is pressed toward the distal end side of the syringe. At this time, the piston may move, causing problems such as leakage of the medical fluid from the syringe.

[0004] In order to solve such a problem, the following Patent Document 1 discloses a plunger assembly for an injector in which a cylindrical hole opened to the proximal end side is provided in a piston (gasket), and a male screw structure is provided at the distal end portion of the plunger rod.

[0005] International Publication No. 2009 / 011214

[0006] In a prefilled syringe, for example, filling of a drug such as a protein preparation into a syringe barrel and vacuum stoppering of a piston are performed in an aseptic environment. When the plunger rod is attached to the piston, it is attached in a normal environment during use.

[0007] However, in the device described in Patent Document 1, when the plunger rod is attached to the piston, the plunger rod is forcibly pushed into the hole in the piston, which may cause the piston to move axially. Also, after the plunger rod is attached to the piston, when the pressure on the plunger rod is released, the piston may move slightly towards the proximal end. Therefore, the risk of the drug solution in the syringe barrel being exposed to the normal environment due to the movement of the piston cannot be ruled out.

[0008] The present invention has been made in view of the above-mentioned problems, and aims to provide a plunger kit, a pre-filled syringe, and a method for manufacturing a pre-filled syringe that can reduce the risk of the drug in the syringe barrel being exposed to the normal environment when the plunger rod is attached to the gasket.

[0009] The present invention is achieved by any one of the following means (1) to (9).

[0010] (1) A plunger kit comprising a gasket fitted to a syringe barrel containing a drug, and a plunger rod fitted to the gasket, wherein the plunger rod has a rod body extending in the axial direction and a fitting portion provided at the tip of the rod body that can be fitted to the gasket, the gasket has a gasket body, an opening opened at the base end of the gasket body, a constricted portion communicating with the opening and extending in the axial direction, and a housing portion continuous with the tip side of the constricted portion that can accommodate the fitting portion, the fitting portion has a shaft portion extending in the axial direction and an engaging portion provided at the tip side of the shaft portion that can engage with the housing portion, the engaging portion is plate-shaped extending along the axial direction and has a connecting portion provided at the tip side and connected to the shaft portion, and a separating portion provided at the base end side of the connecting portion and spaced apart from the shaft portion, the plunger kit.

[0011] (2) The plunger kit according to (1), wherein the mounting portion has two engaging portions.

[0012] (3) The plunger kit according to (1) or (2), wherein when the plunger rod is mounted to the gasket, the tip of the engaging portion is spaced apart from the inner circumference of the gasket.

[0013] (4) The plunger kit according to any one of (1) to (3), wherein the plate-shaped engaging portion is configured to be spaced apart from the shaft portion as it approaches the base end.

[0014] (5) The plunger kit according to any one of (1) to (4), wherein the plate-shaped engaging portion is curved so as it moves toward the base end, it moves away from the shaft portion.

[0015] (6) The plunger kit according to any one of (1) to (5), wherein the plate-shaped engaging portion is configured such that, in a plan view, its width narrows toward the base end.

[0016] (7) The plunger kit according to any one of (1) to (6), wherein when the plunger rod is mounted on the gasket, the portion of the engaging portion closer to the base end than half of the axial length abuts against the constricted portion.

[0017] (8) A pre-filled syringe comprising a plunger kit as described in any one of (1) to (7), and the syringe barrel.

[0018] (9) A method for manufacturing a prefilled syringe as described in (8), comprising: preparing the syringe barrel with the gasket attached and the plunger rod; and attaching the plunger rod to the gasket to obtain the prefilled syringe.

[0019] According to the present invention, when the plunger rod is attached to the gasket, the engaging portion of the plunger rod elastically deforms and is moved toward the tip while being biased radially inward. When the engaging portion exceeds the constricted portion, the inward bias of the engaging portion is released, and the engaging portion is housed in the housing portion. Therefore, the plunger rod can be attached to the gasket with little force, and axial movement of the gasket can be suppressed. Consequently, the risk of the drug in the syringe barrel being exposed to the normal environment when attaching the plunger rod to the gasket can be reduced.

[0020] This is a front view of the prefilled syringe according to this embodiment, showing the plunger rod before it is attached to the gasket. This is a cross-sectional view of the prefilled syringe according to this embodiment, showing the plunger rod before it is attached to the gasket. This is a front view of the prefilled syringe according to this embodiment, showing the plunger rod attached to the gasket. This is a cross-sectional view of the prefilled syringe according to this embodiment, showing the plunger rod attached to the gasket. This is a front cross-sectional view showing the gasket. This is a side view showing the plunger rod attachment part of the prefilled syringe according to this embodiment. This is a cross-sectional view along line 7-7 in Figure 6. This is a cross-sectional view along line 8-8 in Figure 6. This is a cross-sectional view along line 9-9 in Figure 6. This is a cross-sectional view along line 10-10 in Figure 6. This is a plan view showing the plunger rod attachment part of the prefilled syringe according to this embodiment. This is a partially enlarged view showing the process of attaching the plunger rod to the gasket. This is a partially enlarged view showing the state when the plunger rod is attached to the gasket.

[0021] Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the drawings. The embodiments shown herein are illustrative examples for embodying the technical idea of ​​the present invention and do not limit the present invention. Furthermore, all other implementable forms, examples, and operational techniques that can be conceived by those skilled in the art without departing from the spirit of the present invention are included in the scope and spirit of the present invention, as well as in the claims and their equivalents.

[0022] Furthermore, the drawings attached to this specification may be schematically represented with changes to scale, aspect ratio, shape, etc., from the actual object for the sake of illustration and ease of understanding, but these are merely examples and do not limit the interpretation of the present invention.

[0023] In the following explanation, ordinal numbers such as "1st" and "2nd" are used, but unless otherwise specified, they are used for convenience and do not prescribe any particular order.

[0024] The configuration of the pre-filled syringe 100 according to this embodiment will be described below with reference to Figures 1 to 13. Figure 1 is a front view of the pre-filled syringe 100 according to this embodiment, showing the state before the plunger rod 30 is attached to the gasket 20. Figure 2 is a cross-sectional view of the pre-filled syringe 100 according to this embodiment, showing the state before the plunger rod 30 is attached to the gasket 20. Figure 3 is a front view of the pre-filled syringe 100 according to this embodiment, showing the state after the plunger rod 30 is attached to the gasket 20. Figure 4 is a cross-sectional view of the pre-filled syringe 100 according to this embodiment, showing the state after the plunger rod 30 is attached to the gasket 20. Figure 5 is a front cross-sectional view showing the gasket 20. Figure 6 is a side view showing the mounting portion 33 of the plunger rod 30 of the pre-filled syringe 100 according to this embodiment. Figure 7 is a cross-sectional view along the line 7-7 in Figure 6. Figure 8 is a cross-sectional view along the line 8-8 in Figure 6. Figure 9 is a cross-sectional view taken along the line 9-9 in Figure 6. Figure 10 is a cross-sectional view taken along the line 10-10 in Figure 6. Figure 11 is a plan view showing the mounting portion 33 of the plunger rod 30 of the pre-filled syringe 100 according to this embodiment. Figure 12 is a partially enlarged view showing the process of attaching the plunger rod 30 to the gasket 20. Figure 13 is a partially enlarged view showing the state after the plunger rod 30 has been attached to the gasket 20.

[0025] A pre-filled syringe 100 according to this embodiment will now be described. In the following description, the side of the plunger rod 30 that is attached to the gasket 20 (left side in Figure 1) will be referred to as the tip side, and the side opposite the gasket 20 in the axial direction (right side in Figure 1) will be referred to as the base end side.

[0026] As shown in Figures 1 to 4, the pre-filled syringe 100 includes a syringe barrel 10, a gasket 20, and a plunger rod 30. The syringe barrel 10 and gasket 20 constitute the syringe kit 110. The gasket 20 and plunger rod 30 constitute the plunger kit 120.

[0027] The syringe barrel 10 is formed in a cylindrical shape with a lumen that connects the tip and the base. A drug solution chamber 11 for containing the drug solution is formed in the lumen of the syringe barrel 10, extending axially. The drug solution chamber 11 is open at the base end, and is configured so that a gasket 20 and a plunger rod 30 can be inserted into it.

[0028] A nozzle 12 is formed at the tip of the chemical solution storage chamber 11, and a connector 14 is attached to the outer circumference of the nozzle 12. A cap 15 is screwed onto the connector 14. The tip opening of the nozzle 12 is sealed by the cap 15. That is, the tip side of the chemical solution storage chamber 11 is sealed by the cap 15. A gasket 20 is inserted into the chemical solution storage chamber 11 so as to be axially slidable. The base end of the chemical solution storage chamber 11 is sealed by the gasket 20.

[0029] As shown in Figure 5, the gasket 20 has a gasket body portion 21, a tip-side rib 22, a base-side rib 23, and a recess 24. The gasket 20 is made of, for example, a medical-grade rubber material. The gasket 20 is pressed into the syringe barrel 10 at the pressing position by vacuum pressing.

[0030] The gasket 20 is vacuum-sealed into the syringe barrel 10 in a sterile environment. Therefore, immediately after vacuum sealing, the area beyond the gasket 20 is in a normal environment, while the area beyond the gasket 20 is in a sterile environment. After vacuum sealing, the area of ​​the normal environment and the area of ​​the sterile environment within the drug solution chamber 11 of the syringe barrel 10 may change due to the axial movement of the gasket 20 toward the tip, such as when the plunger rod 30 is attached.

[0031] The gasket body portion 21 is formed in a cylindrical shape and is smaller than the inner diameter of the chemical solution chamber 11. The gasket body portion 21 is housed in the syringe barrel 10. A recess 24 is formed inside the gasket body portion 21.

[0032] The tip-side rib 22 is ring-shaped and encircles the radial outer circumference of the gasket body 21, and is formed on the tip side of the gasket body 21. The tip-side rib 22 is formed to be larger than the inner diameter of the chemical solution containment chamber 11.

[0033] The base end rib 23 is ring-shaped and encircles the radial outer circumference of the gasket body 21, and is formed on the base end side of the gasket body 21 than the tip end rib 22. The base end rib 23 is formed to be larger than the inner diameter of the chemical solution chamber 11. The base end rib 23 is positioned axially apart from the tip end rib 22 with respect to the gasket body 21.

[0034] The recess 24 is formed inside the gasket 20 and has an opening at its base end. The mounting portion 33 of the plunger rod 30 is inserted into the recess 24 (see Figures 12 and 13). The recess 24 has an opening 24a that opens at the base end of the gasket body 21, a constricted portion 24b that communicates with the opening 24a and extends axially, and a housing portion 24c that is continuous with the tip of the constricted portion 24b and can accommodate the mounting portion 33 of the plunger rod 30.

[0035] The opening 24a is formed with an inner diameter (see Figure 12) that is approximately the same as the outermost diameter of the engagement portion 33b of the plunger rod 30 along the axial direction, and is formed in a funnel shape that extends for a predetermined length in the axial direction and then narrows towards the constricted portion 24b. The opening 24a guides the mounting portion 33 of the plunger rod 30 into the housing portion 24c.

[0036] The constricted portion 24b is formed with a smaller diameter than the opening 24a. The constricted portion 24b extends axially with a substantially constant inner diameter. By maintaining a constant inner diameter, the engaging portion 33b of the mounting portion 33 can smoothly pass through the constricted portion 24b. The inner diameter of the constricted portion 24b is configured to be smaller than the outermost diameter of the engaging portion 33b of the plunger rod 30. Therefore, when the plunger rod 30 is mounted on the gasket 20, the engaging portion 33b of the plunger rod 30 is biased radially inward and moved toward the tip.

[0037] The housing portion 24c is formed in communication with the tip side of the constricted portion 24b and houses the engaging portion 33b of the plunger rod 30 (see Figures 12 and 13). The housing portion 24c is formed such that, when the engaging portion 33b is housed, the base end 33x of the engaging portion 33b abuts against the inner circumferential surface (see Figure 13).

[0038] Because the gasket 20 has the configuration described above, when the plunger rod 30 is attached to the gasket 20, the engaging portion 33b is inserted from the opening 24a, passes through the constricted portion 24b, and is housed in the housing portion 24c. At this time, when the plunger rod 30 is attached to the gasket 20, the engaging portion 33b of the plunger rod 30 is biased radially inward and moved toward the tip. When the engaging portion 33b exceeds the constricted portion 24b, the inward bias of the engaging portion 33b is released, and the base end 33x of the engaging portion 33b engages with the housing portion 24c. For this reason, the gasket 20 can maintain the state in which the engaging portion 33b is housed in the housing portion 24c, and the plunger rod 30 will not easily fall out of the gasket 20.

[0039] The gasket 20 seals the liquid chemical chamber 11 by having its tip rib 22 and base rib 23 contact and tightly adhere to the inner wall of the liquid chemical chamber 11 inside the syringe barrel 10. As shown in Figure 2, the liquid chemical is contained at the tip end of the gasket 20, with a predetermined space in between.

[0040] As shown in Figures 1 to 4 and 6, the plunger rod 30 includes a rod main body 31, a pressing portion 32 provided at a proximal end of the rod main body 31, and a mounting portion 33 provided at a distal end of the rod main body 31.

[0041] The rod main body 31 extends in the axial direction and transmits an input load applied to the pressing portion 32, for example, a load generated by a user's pressing operation on the pressing portion 32.

[0042] A load in the forward direction or the backward direction is input to the pressing portion 32. The plunger rod 30 can slide along the axial direction inside the syringe barrel 10 when the load applied to the pressing portion 32 is transmitted to the rod main body 31.

[0043] The mounting portion 33 is provided at the distal end of the rod main body 31 and can be mounted to the gasket 20.

[0044] As shown in Figure 6, the mounting portion 33 includes a shaft portion 33a extending in the axial direction, an engaging portion 33b provided on the distal end side of the shaft portion 33a and engageable with the accommodating portion 24c, and an abutting portion 33c provided on the proximal end side of the shaft portion 33a and abutting against the proximal end of the gasket 20.

[0045] The engaging portion 33b is provided on the distal end side of the shaft portion 33a. A wing portion 33f is provided on an outer circumference of the proximal end side of the shaft portion 33a to prevent stress concentration between the abutting portion 33c and the shaft portion 33a.

[0046] As shown in Figures 6 to 11, the engaging portion 33b is in a plate shape extending along the axial direction. As shown in Figure 6, the engaging portion 33b includes a connecting portion 33g provided on the distal end side and connected to the shaft portion 33a, and a spaced portion 33h provided on the proximal end side of the connecting portion 33g and spaced apart from the shaft portion 33a. In this embodiment, two engaging portions 33b are provided at an interval of 180 degrees along the circumferential direction.

[0047] The plate-shaped engaging portion 33b is configured so as to be further spaced apart from the shaft portion 33a toward the proximal end side, as shown in Fig. 6. In other words, the separation distance between the spaced portion 33h and the shaft portion 33a increases toward the proximal end side. According to the plunger kit 120 configured as described above, when the plunger rod 30 is attached to the gasket 20, the engaging portion 33b abuts at a portion further spaced apart from the connecting portion 33g toward the proximal end side, whereby the force generated when the engaging portion 33b is elastically deformed can be reduced. Therefore, since the plunger rod 30 can be attached to the gasket 20 with a weaker force, movement of the gasket 20 in the axial direction can be further suppressed. Accordingly, when attaching the plunger rod 30 to the gasket 20, the risk that the drug in the syringe barrel 10 is exposed to the normal environment can be further reduced.

[0048] Furthermore, the plate-shaped engaging portion 33b is configured to be curved so as to be spaced apart from the shaft portion 33a toward the proximal end side. According to the plunger kit 120 configured as described above, since the plunger rod 30 can be attached to the gasket 20 with a weaker force, movement of the gasket 20 in the axial direction can be further suppressed. Accordingly, when attaching the plunger rod 30 to the gasket 20, the risk that the drug in the syringe barrel 10 is exposed to the normal environment can be further reduced.

[0049] Furthermore, as shown in Figs. 7 to 11 in a plan view, the plate-shaped engaging portion 33b is configured such that the width thereof becomes narrower toward the proximal end side. According to the plunger kit 120 configured as described above, since the plunger rod 30 can be attached to the gasket 20 with a weaker force, movement of the gasket 20 in the axial direction can be further suppressed. Accordingly, when attaching the plunger rod 30 to the gasket 20, the risk that the drug in the syringe barrel 10 is exposed to the normal environment can be further reduced.

[0050] Furthermore, as shown in Figure 13, when the plunger rod 30 is attached to the gasket 20, the tip portion of the engaging portion 33b beyond the base end 33x is spaced apart from the inner circumference of the gasket 20. For example, if the tip portion of the engaging portion 33b beyond the base end 33x is configured to contact the inner circumference of the gasket 20, a greater force will be required when attaching the plunger rod 30 to the gasket 20, causing the gasket 20 to move and creating a risk that the drug in the syringe barrel will be exposed to the normal environment when attaching the plunger rod 30 to the gasket 20. In contrast, since the tip portion of the engaging portion 33b beyond the base end 33x is spaced apart from the inner circumference of the gasket 20, the movement of the gasket 20 can be further suppressed, and the risk of the drug in the syringe barrel being exposed to the normal environment when attaching the plunger rod 30 to the gasket 20 can be further reduced. Note that a configuration in which the tip portion of the engaging portion 33b beyond the base end 33x contacts the inner circumference of the gasket 20 is also included in the present invention.

[0051] Furthermore, as shown in Figure 13, when the plunger rod 30 is attached to the gasket 20, it is preferable that the portion of the engaging portion 33b closer to the base end than half of its axial length contacts the constricted portion 24b. With the plunger kit 120 configured in this way, when the plunger rod 30 is attached to the gasket 20, contact occurs at a portion further away from the connecting portion 33g towards the base end, thereby reducing the force required for elastic deformation of the engaging portion 33b and further suppressing the movement of the gasket 20. Therefore, the risk of the drug in the syringe barrel being exposed to the normal environment when the plunger rod 30 is attached to the gasket 20 can be further reduced.

[0052] Next, the manufacturing method for the pre-filled syringe 100 will be described.

[0053] With the drug solution contained in the drug solution chamber 11 of the syringe barrel 10, the gasket 20 is positioned at the base end and the gasket 20 is fitted by vacuum sealing. The sealing of the gasket 20 is performed in a sterile environment.

[0054] Next, the mounting portion 33 of the plunger rod 30 is inserted into the recess 24 of the gasket 20, and the plunger rod 30 is mounted on the gasket 20. This process is performed in a normal environment. During this process, the mounting portion 33 is mounted while elastically deforming, which suppresses the movement of the gasket 20 toward the tip. Therefore, the plunger rod 30 is mounted on the gasket 20 with almost no movement of the gasket 20 from its mounting position. With this, the assembly of the pre-filled syringe 100 is completed, and the manufacturing process is finished. After the assembly of the pre-filled syringe 100 is complete, the drug solution can be administered to a living organism by performing a predetermined operation.

[0055] As described above, the plunger kit 120 according to this embodiment is a plunger kit 120 that includes a gasket 20 that is attached to a syringe barrel 10 containing a drug, and a plunger rod 30 that is attached to the gasket 20. The plunger rod 30 has a rod body 31 that extends in the axial direction, and a mounting portion 33 provided at the tip of the rod body 31 that can be attached to the gasket 20. The gasket 20 has a gasket body portion 21, an opening 24a that opens at the base end of the gasket body portion 21, a constricted portion 24b that communicates with the opening 24a and extends in the axial direction, and a housing portion 24c that is continuous with the tip side of the constricted portion 24b and can accommodate the mounting portion 33. The mounting portion 33 has a shaft portion 33a that extends in the axial direction, and an engaging portion 33b provided at the tip side of the shaft portion 33a that can engage with the housing portion 24c. The engaging portion 33b is plate-shaped and extends along the axial direction, and has a connecting portion 33g provided at the tip end and connected to the shaft portion 33a, and a separating portion 33h provided at the base end of the connecting portion 33g and spaced apart from the shaft portion 33a. With the plunger kit 120 configured in this way, when the plunger rod 30 is attached to the gasket 20, the engaging portion 33b of the plunger rod 30 is elastically deformed and moved toward the tip end while being biased radially inward. When the engaging portion 33b exceeds the constricted portion 24b, the inward bias of the engaging portion 33b, which was biased radially inward, is released, and the engaging portion 33b is housed in the housing portion 24c. Therefore, the plunger rod 30 can be attached to the gasket 20 with little force, and axial movement of the gasket 20 can be suppressed. Consequently, the risk of the drug in the syringe barrel 10 being exposed to the normal environment when attaching the plunger rod 30 to the gasket 20 can be reduced.

[0056] Furthermore, the manufacturing method of the pre-filled syringe 100 according to this embodiment involves preparing a syringe barrel 10 fitted with a gasket 20 and a plunger rod 30, and then attaching the plunger rod 30 to the gasket 20 to obtain the pre-filled syringe 100. This manufacturing method reduces the risk of the drug inside the syringe barrel being exposed to the normal environment when attaching the plunger rod 30 to the gasket 20.

[0057] It should be noted that the present invention is not limited to the embodiments described above, and can be modified in various ways within the scope of the claims.

[0058] For example, in the embodiment described above, the mounting portion 33 had two engaging portions 33b. However, the mounting portion 33 may have one or more engaging portions 33b.

[0059] Furthermore, in the embodiment described above, the plate-shaped engaging portion 33b was configured to be further separated from the shaft portion 33a as it approached the base end. However, the distance between the plate-shaped engaging portion 33b and the shaft portion 33a may be constant as it approaches the base end.

[0060] Furthermore, in the embodiment described above, the plate-shaped engaging portion 33b was configured to be curved so as it moved further away from the shaft portion 33a towards the base end. However, the plate-shaped engaging portion 33b may also be configured to be straight so as it moves further away from the shaft portion 33a towards the base end.

[0061] Furthermore, in the above-described embodiment, the plate-shaped engaging portion 33b was configured such that its width narrowed towards the base end when viewed from above. However, the plate-shaped engaging portion 33b may be configured such that its width is constant or increases towards the base end when viewed from above.

[0062] This application is based on Japanese Patent Application No. 2025-050897, filed on 26 March 2025, the disclosures of which are cited in their entirety by reference.

[0063] 10 Syringe barrel, 11 Drug chamber, 12 Nozzle, 20 Gasket, 21 Gasket body, 24 Recess, 24a Opening, 24b Constriction, 24c Storage section, 30 Plunger rod, 31 Rod body, 32 Pressing section, 33 Mounting section, 33a Shaft section, 33b Engaging section, 33g Connecting section, 33h Separating section, 100 Prefilled syringe, 110 Syringe kit, 120 Plunger kit.

Claims

1. A plunger kit comprising: a gasket fitted to a syringe barrel containing a drug; and a plunger rod fitted to the gasket, wherein the plunger rod has: a rod body extending in the axial direction; and a fitting portion provided at the tip of the rod body that can be fitted to the gasket; the gasket has: a gasket body; an opening opening at the base end of the gasket body; a constricted portion communicating with the opening and extending in the axial direction; and a housing portion continuous with the tip side of the constricted portion that can accommodate the fitting portion; the fitting portion has: a shaft portion extending in the axial direction; and an engaging portion provided at the tip side of the shaft portion that can engage with the housing portion; the engaging portion is plate-shaped and extends along the axial direction, and has: a connecting portion provided at the tip side and connected to the shaft portion; and a separating portion provided at the base end side of the connecting portion and spaced apart from the shaft portion.

2. The plunger kit according to claim 1, wherein the mounting portion has two of the engagement portions.

3. The plunger kit according to claim 2, wherein when the plunger rod is mounted to the gasket, the tip end of the engaging portion is spaced apart from the inner circumference of the gasket.

4. The plunger kit according to claim 2, wherein the plate-shaped engaging portion is configured to be further spaced apart from the shaft portion as it approaches the base end.

5. The plunger kit according to claim 4, wherein the plate-shaped engaging portion is curved so as it moves toward the base end, it moves further away from the shaft portion.

6. The plunger kit according to claim 2, wherein the plate-shaped engaging portion is configured such that, in a plan view, its width narrows toward the base end.

7. The plunger kit according to any one of claims 1 to 6, wherein when the plunger rod is mounted to the gasket, the portion of the engaging portion closer to the base end than half of the axial length abuts against the constricted portion.

8. A pre-filled syringe comprising the plunger kit described in claim 1 and the syringe barrel.

9. A method for manufacturing a prefilled syringe according to claim 8, comprising: preparing the syringe barrel with the gasket attached and the plunger rod; and attaching the plunger rod to the gasket to obtain the prefilled syringe.

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