PURPOSE: To suppress the contamination of working gas and suppress the impact noise by the simple constitution by directly connecting a compression space and a buffer space which are partitioned by a reciprocating piston through a connecting circuit and installing a check valve in series with a purifying chamber, into the connecting circuit.

CONSTITUTION: When a crankshaft 2 is revolved by an electric motor 1, a piston 5 moves in reciprocation in a cylinder 6, and a compression space 12 repeats compression and expansion. When the gas pressure in the compression space 12 lowers under the gas pressure in a buffer space 13, gas is introduced into the compression space 12 from the buffer space 13 through a check valve 15, passing through a connecting circuit 14 for directly connecting the both spaces. Therefore, the gas stream forms an unidirectional circuit, and aspiration action is not generated between the both spaces. Further, the gas which is formed by the slidable movements of a piston ring 7 and a cylinder 6 and contaminated by the abrasion powders which drop into the buffer space 13 and float is purified in a purifying chamber 16 formed in the connecting circuit 14.