PURPOSE: To prevent intermediate stage pressure from abnormal rise by mounting a suction valve opening type unloader on a suction valve of a low stage side compressor to operate the unloader when suction pressure or discharge pressure reaches at least a certain pressure.

CONSTITUTION: An unloader 20 is operated by instrumentation air sent through a solenoid valve 21 to forcibly open a suction valve 111 when suction pressure or discharge pressure of a low stage side compressor 11 reaches at least a certain pressure. Namely, by opening the suction valve 111 is placed the low stage side compressor 11 under the idling condition to lower the intermediate stage pressure, and only a high stage side compressor 12 is operated with load to hold normal intermediate stage pressure.