A rotary actuator (10) is provided with a linear activation mechanism (40a) that causes a pinion (80) to rotate, and a cylinder body (12) in which a cylinder hole (28a) is formed. The linear activation mechanism (40a) comprises a rack (42) on which a plurality of teeth (44) that mesh with the pinion (80) are provided, and pistons (48a, 48b). The pistons (48a, 48b) are each provided with a piston main body (50a, 50b) that has a shape corresponding to that of the cylinder hole (28a). Each of the piston main bodies (50a, 50b) comprises a body (52a, 52b) and an extension section (54a, 54b). The ends of the rack (42) are connected to the extension sections (54a, 54b) of the set of pistons (48a, 48b), and a space (73a) shielded from the cylinder hole (28a) is formed between the bodies (52a, 52b) of the set of pistons (48a, 48b). The linear activation mechanism (40b) is also configured similarly.