PURPOSE: To obtain a titled apparatus by which terrestrial heat can be utilized at its maximum, by changing the direction of air flow passing through a pipe buried under the ground by operating a directional control valve.

CONSTITUTION: Both suction pipe 11 and a drain pipe 12 are connected to a pipe 9, which forms a closed cycle and is buried under the ground, by the intermediary of a directional control valve 10. Those pipes 11 and 12 are laid in a building 13. The directional control valve 10 is constituted in such a manner that it can change the direction of air flow in the buried pipe 9, without changing the direction of air flow in the suction pipe 11 and in the drain pipe 12 on the indoor side. When the air in a specified temperature is continuously fed into the buried pipe 9, the rate to utilize the terrestrial heat in a part A is remarkably decreased in the direction of a flow shown by (A), while the rate to utilize heat in a part B is remarkably decreased in the direction of a flow shown by (B). Accordingly, terrestrial heat can be utilized at its maximum, by changing the direction of air flow in the buried pipe 9 by the directional control valve 10 in accordance with lowering of the utilizing rate of terrestrial heat, without changing the direction of air flow in the pipe-lines on the indoor side.