PURPOSE: To conduct air conditioning and mount an air conditioner even on a place on which the air conditioner has been conventionally difficult to be mounted by providing the condensation heat exchanger of a refrigerant circuit in a water tank and conducting a heat exchange with a liquid filled in the water tank.

CONSTITUTION: A refrigerant changed to a high temperature and high pressure gas by a compressor 1 performs a heat exchange in an in-liquid heat exchanger 2 with a drainage from a sink 12 trapped in a drainage trap water tank 6 and the temperature of the refrigerant is decreased. The refrigerant enters an air heat exchanger 4 via a pressure reducing mechanism 3 in the condition of low temperature and low pressure liquid. The air heat exchanger 4 is in a draft circuit and supplied with indoor air by a blower 5. Air conditioning if conducted by a heat exchange between the indoor air and the refrigerant. When a thermal balance is broken by an operational condition, a water temperature in the drainage trap water tank 6 is too raise and heat radiation is insufficient, a solenoid valve 9a is opened by a control mechanism, city water at a low temperature from a city water pipeline is introduced into the drainage trap tank 6 via a city water conduit 10 and a check valve 9b to reduce the water temperature and the thermal balance is restored, thereby a stable operation being continued.