PURPOSE: To eliminate drain in an oil tank so as to dispense with draining work by leading dehumidified discharge air into a suction passage behind a closed suction valve during no-load operation with the suction valve closed so as to form a closed circuit, and circulating the air in this closed circuit.

CONSTITUTION: Dehumidified air led into a suction passage is compressed along with lubricating oil containing moisture, fed from a lubricating oil feeding circuit 6, and moisture contained in the lubricating oil is taken into the dehumidified air. The lubricating oil reduced in moisture is primarily separated by a preseparator PS and recovered into an oil tank OT. Since the lubricating oil in the oil tank OT is circulated from the lower part of the oil tank OT through a lubricating oil circulating circuit 5 and the lubricating oil feeding passage 6, the moisture in the oil tank OT is eliminated by the repetition of this circulating action.