PURPOSE: To increase the refrigerant circulation rate of flow during defrosting by providing a bypass circuit through which high temperature, high pressure refrigerant gas is bypassed to an indoor side heat exchanger, and a bypass circuit leading from a receiver tank to the suction side of a compressor via a capillary.

CONSTITUTION: A first bypass circuit 8 branches off from the inlet of a four-way valve 2 on the suction side of a compressor 1 and is connected between an outdoor side heat exchanger 6 and an expansion valve 4 via a first solenoid valve 7, so that hot gas from the compressor 1 can be directly led to the outdoor side heat exchanger 6 to stimulate defrosting of the heat exchanger 6 when defrosting of the heat exchanger 6 is carried out during heating operation. In this case, a second bypass circuit 11 leading from the receiver tank 5 to the suction side of the compressor via a second solenoid valve 9 and a capillary 10 is formed to send liquid refrigerant stored in a room side heat exchanger 3 and the receiver tank 5 to the suction side of the compressor 1 so that the refrigerant circulation rate of flow is increased to stimulate defrosting.