PURPOSE: To reduce knocking sound of a driven gear to a drive gear by discharging compressed air between the upper surface of a piston in the neighborhood of the upper dead point of a piston and a cylinder head through a suction slit formed in a rotary valve.

CONSTITUTION: Compressed air in a space 2c of a cylinder 2 at the neighborhood of the upper dead point of a piston 4 is exhausted at every rotation of a crankshaft 3 through a suction hole 5d of a cylinder head 5 and suction slit 6d of a rotary valve 6 from a suction port 5b of the cylinder head 5. Thus, the remaining pressure in the space 2c of the cylinder 2 is fairly reduced and knocking sound of a driven gear 12 to a drive gear to reduce running sound of a compressor 1. Also, since positions of suction slit 6d and discharge slit 6e of the rotary valve 6 can be changed freely, timing of suction and discharge can be freely selected.