To provide an underground heat exchanger using a temporary underground continuous wall capable of eliminating the necessity of excavation for embedding heat exchange piping and usable from an initial stage of a construction period.

In the underground heat exchanger 1, the heat exchange piping 12 is embedded in the ground and a heating medium is made to flow in the heat exchange piping 12 to exchange heat. The heat exchange piping 12 is arranged along core materials 112 within the temporary underground continuous wall 11, and is covered with protective materials 13.