A valve body adjusts opening of a supply passage in response to the position in a valve chamber. A pressure-sensitive member is moved in accordance with the pressure difference between two pressure monitoring points, which are located in an external refrigerant circuit. The movement of the pressure-sensitive member affects the position of the valve body such that the compressor displacement is changed to reduce fluctuations in the pressure difference. A solenoid changes force applied to the valve body so that a set pressure difference, which is a reference value for changing the position of the valve body by the pressure-sensitive member, is changed.