To securely enhance thermal conductivity of a filler related to an underground heat exchanger while suppressing manufacturing cost of the underground heat exchanger and improve heat collecting-releasing efficiency of the underground heat exchanger.

The underground heat exchanger for exchanging heat with a ground includes: piping inserted to a hole formed on the ground and having a heating medium made to flow therein; and the filler filled between the hole and the piping. The filler includes long particulates comprising a predetermined volume content of at least any one type of silicon carbide, alumina and blast furnace slag.