To solve a problem that conventionally an indoor unit is an evaporator and an outdoor unit is a condenser in summer and the indoor unit is the condenser and the outdoor unit is the evaporator in winter, and it is basically irrational in efficiency to cool and condense a refrigerant with the hot air of summer and to raise the temperature and evaporate the refrigerant with the cold air of the winter in a heat pump system performing cooling and heating with small quantity of energy in order to maintain comfortable living environment.

The air of which heat is exchanged by feeding the outside air into a heat exchanger of a double pipe structure buried in the earth is applied to a heat exchanger of the outdoor unit of the heat pump type air conditioner equipped with the outdoor unit, and the refrigerant of the heat pump is condensed or evaporated. In the heat exchanger of the double pipe structure, an inner pipe having opened both ends are loosely fitted into an outer pipe having one end sealed and the other end opened, the heat exchanger of the double pipe structure is buried in the earth while the one end of the outer pipe is directed downward, and the air is made to flow into a clearance between the inner pipe and the outer pipe so as to exchange the heat with the the heat of the earth. The outer pipe is buried into the earth to a depth of 5 meters.