To prevent usual time blowoff of air compressed up to intermediate stage pressure at full load operation time by closing a primary side passage part of intermediate stage drain piping, introducing positive pressure air to the secondary side of the drain piping, and arranging a drain trap in the intermediate stage drain piping.

A three-way solenoid valve 13 is installed in drain piping of an intercooler 6, and an intermediate stage drain trap 14 is installed. At full load operation time, the three-way solenoid valve 13 opens between A and B, and intermediate stage drain piping is put in a condition where drain is sent into the intermediate stage drain trap 14 since the primary side 17 and the secondary side 18 lead to each other. At no-load operation time, the three- way solenoid valve 13 is switched by receiving detection by a pressure switch 11, and a part between C and B is opened, and though the primary side 17 of the intermediate stage drain piping is put under negative pressure, blowoff air cooled by a blowoff cooler 16 is sent into the secondary side 18 of the intermediate stage drain piping by piping 19, and the intermediate stage drain trap 14 can be kept in a positive pressure condition.