PURPOSE: To allow the discharge capacity to follow always correctly to the control signal by converting the output signal having a prescribed frequency to the output pulse having a certain amplitude and a prescribed duty ratio.

CONSTITUTION: The discharge capacity is varied by varying the piston stroke by adjusting the communication degree between a suction chamber and a crank chamber by controlling a pressure adjusting valve 60 equipped with an electromagnetic actuator 68. The electric current in pulse form whose amplitude can be controlled flows with a certain duty ratio in a solenoid 63, and the electromagnetic actuator 68 is controlled by the electric current. Though, when the max. amplitude of the pulse of the electric current supplied into the solenoid 63 varies, the supplied electric current once becomes zero, the period is short, and the shift quantity of a valve piece 62 does not return to zero, and electric current is supplied again into the solenoid 63, and the shift quantity of the valve piece 62 corresponds each other with the electric current value supplied into the solenoid 63. Therefore, the pressure adjusting valve 60 is always controlled precisely, and the optimum discharge capacity can be obtained.