To provide an underground heat exchanger capable of stabilizing the liquid surface of a liquid heat transfer medium at the lower part of the heat exchanger during operation, of maintaining a heat transfer area for condensation when used as a condenser, and of preventing return to a compressor as liquid when used as an evaporator.

This underground heat exchanger, which includes a first side channel pipe that passes the heat transfer medium downward or upward and a second side channel pipe connected with the lower end of the first side channel pipe that passes the heat transfer medium to the opposite direction of the first side channel pipe, flows the heat transfer media from the first side channel pipe to the second side channel pipe and vice versa, and collects heat from underground and/or radiates heat to underground; being provided with a liquid reservoir section near the bottom of the underground heat exchanger connected with the channel of the first side channel pipe or that of the second channel pipe. Furthermore, at least one channel pipe out of the two channel pipes has a function to open and close the channel, when the channel of the first side channel pipe or the second side passage pipe returns liquid or gas from the underground to the ground.