PURPOSE: To perform a dehumidifying operation without almost lowering the room temperature by stopping the operation of an indoor blower after the cooling mode operation to form frost on an indoor heat exchanger, and thereafter removing the moisture from the inside of room as drain water by the heating mode operation and exhaustion of air.

CONSTITUTION: A cooling mode operation is first carried out, and after the lapse of a certain set time, the indoor blower is stopped to operate so as to form frost on the indoor heat exchanger. At the same time, the temperature of the indoor heat exchanger is detected. Then, the detected temperature of the indoor heat exchanger is compared with a set temperature T1 in a comparison circuit to discriminate whether or not the detected temperature corresponds to a frosting temperature. In this case, the detected temperature becomes less than the set temperature T1, a four-way valve is switched to the heating cycle and the exhausting device is operated, and indoor air passes through the heat exchanger and exhausted out of the room. Upon this occasion, frost formed on the indoor heat exchanger is molten by a coolant of high temperature and high pressure, and is converted into drain, water and steam, which are exhausted out of the room.