PURPOSE: To relieve a four-way switching valve from a large pressure differential which its operation involves and reduce the loud noise and vibration at the start of defrosting by a method wherein, only in starting the defrosting following control of an overloaded condition and after a condition for the start of defrosting is met, the operation of the compressor is suspended for a time.

CONSTITUTION: When the operation of an outdoor blower 7 is stopped during overloaded heating, the lower pressure declines and frost forms on the outdoor heat exchanger; when the control of the overloaded condition is over and the outdoor blower 7 is set operating, the lower pressure rises and the frost begins to melt. Repetition of the control of an overloaded condition in the manner mentioned above causes growth of freeze on the outdoor heat exchanger 3. The start of the defrosting to dissolve the freeze is conditioned by a specified time T1 to which the total of the time for which the operation of the outdoor blower 7 has been suspended (time for which overloaded conditions have been controlled) should amount. However, generally the high pressure is so high, when integration of the time T reaches T1, that shift of a four-way switching valve 2 in this condition causes generation of a loud noise and vibration, which are disagreeable to the user. For this reason, when the condition for the start of defrosting is met, the operation of the compressor 1 is suspended for a time T2 so as to lower the high pressure; after that, the four-way switching valve 2 is shifted to start the defrosting.