In heating mode, when a thermostat OFF operation is performed, the operating time period of the immediately preceding thermostat ON operation is checked. If this operating time period is equal to or less than a prescribed time period, the number of thermostat ON operations is counted. The number of thermostat ON operations is counted each time that thermostat ON and thermostat OFF operations are repeated. When the number of operations reaches a prescribed number, the system enters a forced defrost mode. When thermostat ON and thermostat OFF operations are repeated in a short cycle, defrosting of the outdoor heat exchanger is carried out. Thus, by forcing the system into defrost mode under conditions in which thermostat ON and thermostat OFF operations are repeated in a short cycle, the adverse effect of a failure to defrost the outdoor heat exchanger under such conditions is eliminated.