A hybrid compressor selectively driven by an engine and an electric motor. The hybrid compressor includes a variable displacement compression mechanism. When the compression mechanism is driven by the motor, the cooling capacity of a refrigeration circuit that includes the hybrid compressor is adjusted by controlling the inclination of the swash plate and the motor speed. In the control procedure, the inclination angle of the swash plate and the motor speed are controlled so that the compression mechanism and the motor are most efficiently operated to achieve the required cooling capacity. Therefore, the hybrid compressor is constantly operated with maximum efficiency.