To improve energy saving effect and functional property while ensuring space saving and the low cost.

In order to construct an underground heat exchanger in which a heat medium R1 is passed through a unit structure 1A buried in the ground, and the heat medium R1 is subjected to the underground heat exchange, another heat medium W independent from the ground heat exchange heat medium R1 is passed, and another heat medium heat exchange section 9 is provided on the unit structure 1A for heat exchanging with the underground heat exchanging heat medium R1 in the buried unit structure 1A.