To provide a method of installing an underground heat exchanger capable of securely improving thermal conductivity to a heating medium when the thermal conductivity is improved by using an expansion effect of a surface area of a pipe wall achieved by a corrugated pipe.

The method of installing the underground heat exchanger for performing heat exchange with a ground includes: an excavation process of forming an excavation hole at the ground; an insertion process of inserting the corrugated pipe serving as a flow passage of the heating medium related to the heat exchange into the excavation hole; and a filling process of filling a filler in a space between the excavation hole and the corrugated pipe. In the filling process, air gap suppression processing for suppressing the formation of air gaps in the vicinity of the outer peripheral face of the corrugated pipe is performed.