A cap unit (10) for a container (38) has a cap (20) and an annular skirt (12). The skirt (12) is mounted in the container (38) defining an aperture, the cap (20) being movable between a first position and a second position. In the first position the cap (20) covers the aperture and seals it, and in the second position the cap uncovers the aperture, the cap (20) being journalled pivotably around an axis (26) perpendicular to the plan defined by the aperture and placed outside the aperture so that the cap may swing between the first position and the second position. Furthermore, the cap (20) and the skirt (12) have respective claw-formed catching means (14, 16, 18) which are adapted to fix the cap (20) in relation to the skirt (12) and allow the cap (20) to swing from a first to a second position. The cap unit (10) has driving means (34) for production of swinging of the cap (20) and the cap unit (10) has an inflatable sealing ring (22) mounted in the cap (20) for sealing against the upper surface of the skirt (12) when the cap (20) is in its first position, or alternatively mounted in the skirt (12) for sealing against the opposite surface of the cap (20).