The circulation circuit used for an air-conditioning system for a vehicle. The circulation circuit is provided with a variable capacity type compressor, a condenser, a variable orifice device, an evaporator 4, and an accumulator 5 interposed between the evaporator 4 and the compressor 1. The accumulator 5 is used also as a silencer for preventing noise caused due to the self-excited vibration of the internal mechanism of the compressor. The variable orifice device is used as a pressure reducing device and is capable of varying the cross-sectional area of a restricted path provided therein in response to the temperature of the coolant passing through the restricted path. Therefore, it is possible to change the pressure reducing ability in accordance with the change of the flowing rate of the coolant.