To prevent a foreign substance such as dust from sticking on a discharge port and a discharge valve part by opening the other end of an opening passage toward a discharge valve, and jetting a part of air fed in the opening passage at opening an unload valve toward the discharge valve.

By opening an unload valve at being unnecessary for compressed air, the inside of a cylinder part and a discharge port 8 are communicated to each other. As a result, resistance for reciprocating a piston in the cylinder part is reduced, and useless power consumption can be restrained. A part of air fed into the opening passage 12 at opening the unload valve is jetted toward a discharge port 6 and a discharge valve 10, hence a foreign substance such as dust is hardly stuck to the discharge port 6 and the discharge valve 10 part, and even if a foreign substance is stuck to them, it can be blown off.