PURPOSE: To improve a property of acceleration of an engine by a constitution wherein a capacity varying member is moved to a position where an air compressor is operated with a minimum capacity, on the basis of an OFF signal of a drive switch of the compressor or an ignition key, on the occasion of stopping a vehicle.

CONSTITUTION: When a drive switch 27 of an air conditioner or an ignition switch 28 is turned OFF in order to stop a vehicle for a long time, CPU 18 controls a solenoid valve 11 immediately to drive it with a duty ratio 1, on the basis of an OFF signal of the switch, and makes a pressure in a crank chamber 12 equal to the one in a discharge chamber 14 quickly. Therefore the angle of a swinging slant plate is changed to bring about a state wherein the capacity of a variable capacity compressor 1 becomes minimum. In the case when a drive of the vehicle is started after a long-time stop, accordingly, the variable capacity compressor 1 is always started to operate from the state of the minimum capacity. Therefore a load to be put on an engine becomes minimum and a property of acceleration of the vehicle can be improved.