PURPOSE: To prevent the occurrence of drain by regulating the compressed air flowing through a conducting tube to the proximity of the atmospheric pressure when the temperature in a tube conducting between the low pressure side compressing section and the high pressure side compressing section is low.

CONSTITUTION: A temperature detector 15 will detect the temperature in a conducting tube 9, and when said temperature is below predetermined level a switch section 15A will close and the inlet 18 of a sluice valve 16 is held in the choked state. In the suction process of the low pressure side compressing section 5, the air supplied through an atmospheric air take-in tube 7 will pass only through a throttle path 19 thus to limit the flow. Since the compressed air having the pressure near to the atmospheric pressure will pass through the conducting tube 9, the drain will not occur in the compressed air. Said compressed air is led as it is to the high pressure side compressing section 6, then further compressed to flow through a delivery tube 10 into a tank 1.