PURPOSE: To suck oil from other compressors to recover the oil level by interrupting the normal operation to perform an operation which is to produce a differential pressure between closed vessels of compressors.

CONSTITUTION: When the operation periods of time of compressor 1, 2 and 3 reach an operation period of time stored in an oil equalizing operation decision device 90, normal operation is interrupted and switched to an oil equalizing operation. The oil equalizing operation instruction is input to a compressor capacity decision device 55 and a normal operation instruction to an inverter 52, on-off switches 53 and 54 is stopped. Simultaneously, the oil equalizing operation decision device 90 outputs an operating frequency instruction for the inverter-controlled compressor 1 to the inverter 52 through an output device 56 so that one among compressors 1, 2 and 3, which requires makeup oil, is operated at a larger capacity than that of the other compressors, and an on-off instruction for the on-off controlled compressors 2 and 3 to the inverter 52 and the on-off switches 53 and 54 through output device 56, 57 and 58. As a result, the compressors 1, 2 and 3 are operated based on the oil equalizing operation program.