To provide an underground heat exchanger for improving performance of execution of installation work of the underground heat exchanger and reducing installation cost.

The underground heat exchanger performing heat exchange with a ground includes: a flexible resin corrugated pipe arranged within an excavation on the ground; a discharge port for discharging a heating medium to inside of the corrugated pipe; an emission port for emitting the heating medium which has undergone heat exchange with the ground from the corrugated pipe; and a filler filled between the excavation and the corrugated pipe. The corrugated shape of the corrugated pipe is set to be a spiral shape using a pipe axis of the corrugated pipe as a central axis. One pipe end of the corrugated pipe is sealed by a cap member screwed to a part of the corrugated shape at the pipe end serving as a screw part.