To economically construct a geothermal air conditioning system exchanging heat with ground heat.

In this air conditioning system constituted by burying a tubular body in the ground to exchange heat between a fluid inside of the tubular body and the ground heat, an underground tube 1 is composed of a triple-tube of an inner tube 11, an intermediate tube 12 and an outer tube 13, an intermediate layer is vacuum and has a heat insulating property, and a liquid is made to flow into the inside of the outer tube 13 at a terminal end section above ground, and turn down at a tip section in the ground to be returned on ground from the inner tube 11. As a length of the underground tube 1 is determined to a length enough to exchange heat, and it is unnecessary to make the underground tube penetrate above ground at an opposite side, the geothermal air conditioning system can be economically constructed.

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