PURPOSE: To make it possible to completely perform defrosting under any external air condition by detecting the temperature of the outdoor heat exchanger, comparison calculating the detected temperature with respect to a standard temperature lowering line incorporated in the controller, and causing the controller to judge whether the defrosting operation should be started again or the space heating operation should be continued.

CONSTITUTION: A temperature detector 8 is provided in a suction side pipeline of an outdoor heat exchanger 1 to detect a temperature T0 of the outdoor heat exchanger 1 at a starting time point after returning to the space heating operation and the temperature T' thereof after lapse of a time period t'0. The detected value is sent to a controller 9. When the temperature gradient (f) of the outdoor heat exchanger 1 with respect to a set value f1 of an inclination straight line experimentally determined is large, it is judged that no frost remains, the space heating mode operation is continued. When (f) is smaller than f1, it is judged that frost remains, and the defrosting operation is again carried out. The defrosting operation is carried out for a time period t2, and then the operation returns back to the space heating mode. Thereafter, similar judgements are carried out. For this reason, the front residual of the outdoor heat exchanger is removed without being subjected to external air conditions and hence the space heating capacity is improved.