PURPOSE: To prevent an occurrence of drain by raising oil temperature as quickly as possible under keeping a compressed gas path from becoming excessively high in pressure through controlling the quantity of gas to be sucked in a compressor body.

CONSTITUTION: A suction filter 4 is connected to suction side of a compressor body 1 by a motor 2 through an electromagnetic suction valve 3, and a discharge pipe 5 extending from discharge side of the body 1 is kept open over the oil surface. Next, a pressure switch 19 is connected to a storage tank 8, the switch 19 being of normally-closed type to close at pressure not higher than a given minimum pressure P1 in the tank 8 and open at a given maximum pressure P2 (P2>P1). Then in an oil tank 6, there is provided an auxiliary pressure switch 20 which is actuated according to oil temperatures, and further provided are a temperature switch 21 actuated according to oil temperatures and a compressed gas release valve 22. The above constitution is available in preventing an occurrence of drain.