PURPOSE: To increase the operation efficiency of a multivane compressor by setting a discharge valve in normally opened state in a specific operation condition and allowing the valve to ON-OFF-operate in other operation conditions.

CONSTITUTION: In cooling operation with a relatively low compression ratio, a discharge valve 9 is maintained in open state, and the discharge gas is discharged smoothly from a discharge port 3 free from any resistance, and the loss of the discharge system is reduced markedly. As the compression ratio is relatively low at this time, the amount of counter current of the discharge gas is relatively small even when the discharge valve 9 is in open state, and as a whole, the loss is reduced, and the efficiency of the compressor is improved. On the other hand, when the compression ratio is high, the discharge valve 9 ON-OFF-operates as normal check valve. Accordingly, the discharge system loss due to the pressence of the discharge valve 9 is generated, but the counter flow of discharge gas due to the increase of compression ratio is certainly prevented, and as a result of this, total loss can be reduced.