PURPOSE: To rise the temperature of the hot air diffused into the room up to the confortable level by energizing the first and second heaters for defrosting while the energization in the normal heating operation is only confined to the first heater with a small capacity.

CONSTITUTION: In the normal heating operation, a relay 8 for the first heater is energized to make up for the shortage of heat, which was previously caused during the heating with the heat pump, by heat from the first heater. When frosting in an outdoor exceeds a given level during the normal heating, a defrosting thermostat 12 is switched to energize a relay 11 whereby a four-way reversing valve is deenergized, so that the refrigerant is switched to the cooling side from the heatihg side. Thus, the freezing cycle is changed from the heating to the cooling, and cool air is diffused into the room. But at the same time, a relay 13 for the second heater 2 is energized allowing the heater to generate heat. Accordingly, the strop in the room temprature is compensated for by heat from the second heater.