PURPOSE: To enhance defrosting performance when outdoor air temperature is low by constituting an air conditioner in such a manner that, in a defrost mode with outdoor air temperature lower than a predetermined temperature, indoor air is circulated, dehumidified, and heated for a preset period and then sent out from a defroster blow-off port.

CONSTITUTION: When a mode is switched to "defrost", a microcomputer 22 compares the temperature 14 of outdoor air taken into an air conditioner with a predetermined atmospheric temperature. If the outdoor air temperature 14 is higher than the predetermined temperature, the computer give instruction for intaking of outdoor air. If the temperature 14 is lower, however, for a preset period from the time of mode switching, indoor air is taken in from an indoor air take-in port 2, and, with water cock 30 kept open, a heater core 7 kept operating, and a compressor kept driving, the indoor air 2 dehumidified by cooling by means of an evaporator 5, heated by the heater core 7, and then sent out from a defrost blow-off port 13. With this constitution, highly efficient defrosting can be achieved even when the outdoor air temperature is low.