Underground heat exchanger

[Problem] To provide an underground heat exchanger capable of ensuring an improvement in heat efficiency. [Solution] The present invention is provided with: a flexible bottomed cylindrical bag body 5 which is housed in a housing hole 3 vertically provided in a ground 2 and extends vertically; and an outer pipe 10 which is housed in the housing hole 3 and extends vertically along the outer surface 6 of the bag body 5, and which has a lower end 7 communicating with the lower end of the bag body 5. When the bag body 5 comprising a curable resin is in an expanded state, the outer surface 6 of the bag body 5 covers the inner wall 11 of the housing hole 3 in close contact therewith. The bag body 5 is cured in such a covering state. The lining cylindrical body 15 formed by curing forms a liquid storage tank 19 for storing heat transfer liquid 17. The outer pipe 10 is held between the outer surface 6 of the bag body 5 and the inner wall 11.