PURPOSE: To raise the suction temperature of a compressor before starting defrosting operation to heat the compressor and shorten a defrosting time utilizing the heat of the compressor by a method wherein the amount of ventilation is controlled by detecting the frosting of an outdoor heat exchanger and, thereafter, the defrosting operation is started.

CONSTITUTION: The amount of ventilation is controlled by detecting the frosting of an outdoor heat exchanger 3 and, thereafter, defrosting operation is started. When a temperature sensor 11, provided in the outdoor heat exchanger 3, detects a temperature lower than -12°C, for example, the preparation stage of defrosting is effected during a predetermined period of time. In the preparation stage of defrosting, the amount of ventilation of an indoor fan device 9 is reduced to reduce the amount of heat dissipation in an indoor side and the amount of ventilation of an outdoor fan device 10 is increased to increase heat absorption in an outdoor side while the capacity of a compreesor 1 itself is increased to increase the tempera ture of the compressor 1 itself. Then, the defrosting operation is effected when the preparation stage of defrosting has elapsed the predetermined period of time and an opening and closing valve 7, provided in a bypass circuit 8, is opened whereby high-temperature and high-pressure refrigerant gas, discharged out of the compressor 1, flows into an indoor heat exchanger 5 and the defrosting operation is effected while flowing a part of the same gas into the bypass circuit 8 to effect room heating.