An air conditioning system for a vehicle that is driven by a vehicular power source. The system has a compressor selectively operable by the vehicular power source and an electric motor. The electric motor is used as a drive force of the compressor when the vehicular power source is in a non-operative state. The compressor compresses refrigerant gas introduced into a suction chamber from an external refrigerant circuit. A displacement of the compressor is variable, based on a differential pressure. The compressor has a control valve that is disposed on a refrigerant passage communicating with the crank chamber. The control valve has a valve plunger for changing an opening size of the control valve to adjust pressure in the crank chamber. The air conditioning system comprises pressure sensing member, actuator, and controller. The pressure sensing member is disposed in the control valve and applies biasing force to the plunger based on pressure in the external circuit. The biasing force is applied to cancel change of the pressure in the external circuit. The actuator is disposed in the control valve and applies reverse force against the biasing force to the plunger. The plunger is moved to increase the displacement by the reverse force. The controller controls the actuator to increase the reverse force in steps by a magnitude that is small enough that the electric motor is able to stably drive the compressor.