A variable displacement compressor air conditions a compartment and includes a suction chamber, a discharge chamber and a crank chamber. A controller controls the pressure in the crank chamber to vary the compressor displacement. Two pressure monitoring points are located in a refrigerant circuit. The pressure in the crank chamber is controlled by a control valve. The control valve operates based on the pressure difference between the monitoring points such that a target pressure difference is maintained. A temperature sensor monitors the temperature of the compartment. A detection circuit compares the monitored temperature with reference values. When the monitored temperature surpasses one reference value or falls below another, the detection circuit outputs a detection signal. When receiving the detection signal, a pressure difference changer gradually increases or decreases the target value of the pressure differences accordingly.