To most efficiently collect and utilize a maximum amount of geothermal energy for snow melting, heating, hot water supply and the like without the fear of environmental destruction and pollution such as causing subsidence of ground and lowering of groundwater level and encountering a difficulty in liquid waste disposal and management.

An underground heat exchanger comprises a plurality of heat exchanging concentric duplex tubes embedded vertically under the ground at intervals of at least 5m and having an inner tube 1b inserted concentrically into an outer tube 1a so that a heat medium M is fed downward and reversed to be conducted upward between the inner tube and the outer tube which receives geothermal energy. Preferably, one heat medium feeding pump is provided for four to six concentric duplex tubes which share a heat medium flowrate of 20 to 25l/min.