PURPOSE: To shorten the time required for defrosting operation, by operating a fan for a heat exchanger on the side of a heat source by carrying on defrosting cycle for a predetermined while after completion of defrosting of the heat exchanger on the side of the heat source, and thereby removing frost and water droplets promptly.

CONSTITUTION: In space heating operation, a switch 15a is closed and a switch 15b is thrown to the left, so that a contact 21a is closed through energization of a relay 21. Resultantly, a four-way valve coil 20 and contactors 18, 19 for a fan motor and a compressor are energized for starting space-heating operation. Here, if the amount of frost attached to a heat exchanger on the side of a heat source becomes greater than a predetermined level, a contact 16 for a defrosting thermostat is opened, so that current supply to the coil 20 and relay 21 is interrupted. Resultantly, the four-way valve is switched and current supply to the contactor 18 is interrupted for stopping operation of the fan motor and carrying out defrosting operation. When the defrosting operation is completed and the contact 16 is closed, the contact 21a is closed through energization of the relay 21 and the fan motor is set into operation. At the same time, current is supplied to a timer 25, and after passing of a predetermined time, a contact 25a is closed for supplying current to the coil 20 and thereby switching operation of the apparatus to space-heating operation.