To provide an underground heat exchanger including a pipe formed of an inexpensive and well-moldable material having high heat conductivity.

The underground heat exchanger comprises the U-shaped pipe as a heat medium flow path embedded in a ground for heat exchange between an underground material and heat medium. Particularly, the U-shaped pipe has a highly heat conductive resin containing a material having higher heat conductivity than the resin. Herein, the material having higher heat conductivity than the resin is a heat conductive filler joined with a low melting point alloy. Thus, the pipe has higher heat conductivity than a general resin such as cross-linked polyethylene to improve the efficiency of collecting/releasing the heat of the heat medium circulating in the pipe from/to the material around the pipe.