To provide an underground heat exchanger in which a forward path and a backward path are not individually deformed, a form is easily maintained and an excessive stress does not act on a communicating path.

The underground heat exchanger A comprises a vertically extending main body member 10, a bottom member 20 (end member) provided in the lower end (end) of the main body member 10 and a cover member 30 (base end member) provided on an upper end (base end). The main body member 10 includes a main tube (tube) 11 and a main partition wall 12 with a cross shape in section provided in the main tube 11 to form four longitudinally extending through holes 10a to 10d. The through holes 10a and 10d are connected to the inlet pipes 34A and 34B (injection passages) of the cover member 30 so that they serve as the forward paths of a primary heating medium R1. The through holes 10b and 10c are connected to outlet pipes 35A and 35B (discharge passages) so that they serve as the backward paths. In the bottom member 20, semicircular chambers 20a and 20b (communicating passages) are formed. The through holes 10a and 10b as the forward paths communicate with the through holes 10b and 10c as the backward paths through the semicircular chambers 20a and 20b.