PURPOSE: To develop a difference of water pressure between the lower end section of a water pumping pipe and the hole section of a returning pipe to circulate naturally the water and also save power required for heating the water by keeping the inside and outside of a double walled pipe connected to a heat pump from contacting the atmosphere.

CONSTITUTION: The end of a water pumping pipe 4 with double walls is placed in a underground water layer 9 in a deep stratum and a return water pipe 5 is welded to the water pumping pipe 4 and its end is sealed. Water storage pumped up by a water storage pump 6 positioned at an intermediate position of the water pumping pipe 4 goes into a heat pump 1, and the water heated there is sent to a group of heat pipes to a heat plate 3 and then returned again to the heat pump 1 to be circulated. The pumped water storage is returned to a stratum that has high permeability to water and is located near the ground surface. Because at this time the pumped water storage does not come in contact with the air, the water circulates without driving the water storage pump 6.