PURPOSE: To ensure a sufficient amount of gas discharged for a short time by providing a discharging recess on the lowest portion of slant surface of a piston carrier and discharging compressed gas when said recess coincides with a discharging port opening to a cylinder.

CONSTITUTION: When one end edge of a discharging recess 12 is opposed to a discharging hole 20 as a piston carrier 9 is rotated, compressed gas begins to be discharged, and when the other side edge is opposed to the discharging hole 20, the discharge is completed. A first slant surface 11 then varies its position above and below the discharging hole 20 to reduce a distance in which the discharging hole 20 is opened so that sufficient discharge can be carried out for a short time.