To provide a method for installing an underground heat exchanging unit capable of performing short-circuited heat exchange with the peripheral ground or underground water to thereby improve heat exchanging efficiency for collecting heat or radiating heat, relaxing restrictions on the number of installed heat exchanging pipes, quickly and easily performing the installing work of the heat exchanging pipes even at a deep depth and reducing construction cost, and preventing damage to the heat exchanging pipes.

(a) An excavated hole 11 having an excavated diameter larger than that in a normal method for inserting only a pile body in the ground is excavated in advance by an auger. (b) A pile body 12 of a prefabricated pile formed of concrete is inserted in the excavated hole 11. (c) After installing the pile body 12, an underground heat exchanging unit 14 assembled in a cylinder cage shape is inserted in the excavated hole 11 having a larger diameter than that by the normal method at an outer peripheral part of the pile body 12 (a portion filled with a foot protection liquid or a periphery-of-pile fixing fluid 13) using an installed pile body 12 as a guide to a predetermined depth.