PURPOSE: To supplement the capacity of subterranean heat and maintain the heat exchanging efficiency of a heat exchanger by burying a water discharging pipe for flowing houehold waste water around the heat exchanger for effecting heat exchange between subterranean heat.

CONSTITUTION: The heat exchanger 1, bent into U-shape substantially, is buried into ground oand a pluralit of drain pipes 2 are arranged in parallel substantially to the heat exchanger 1 around the heat exchanger 1. A plurality of waste pipes 2 are joined into one and is capable of being switched into a hot waste pipe 4 or a cold waste pipe 5 through a changeover valve 6. In summer, in case air (a) is cooled, the subterranean temperature is increased and the capatiy of subterranean heat is reduced, therefore, the cold waste water is fowed through the waste pipe 2 previously to form a cold accumulating area (b) and packup the capacity of subterranean heat and on the contray, in winter, the hot waste water is flowed through the waste pipe 2 to packup the capacity of geothermal temperature.