An air-conditioning system has a refrigerant circuit. The circuit includes a condenser, a expansion valve, an evaporator and a variable displacement compressor. The system has a pressure difference detector. The pressure difference detector detects the pressure difference between two pressure monitoring points located on the refrigerant circuit. A control valve and a controller control the displacement of the compressor in accordance with the pressure difference detected by the pressure difference detector. This permits the displacement of the compressor to be quickly changed.