To provide a device for utilizing underground heat with a simple structure without using power and a special operating fluid.

A double pipe in which an inner pipe and an outer pipe 2 are replaced on the way and having opening sections 6 of the inner pipe and the outer pipe 2 at a lower section, is buried in the ground from an underground heat utilizing section 1. As an underground temperature of a place near the ground surface is also lowered when an outside air temperature is lowered, a temperature of the air or water of a low temperature passage section 8 kept into contact with the ground through a pipe wall is lowered, the air or water is contracted, and a specific gravity is increased. The air or water enters into the low temperature passage section 8 from an inlet 12 of the low temperature passage section for the contraction of the air or water in the low-temperature passage section 8. As an atmospheric pressure or a water pressure of the low temperature passage section 8 becomes higher than that of a high temperature passage section 9, at the lower opening sections 6, the air or water enters from the low temperature passage section 8 to the high temperature passage section 9. In accompany therewith, the air or water is discharged from the high temperature passage section 9 to the underground heat utilizing section 1 at an upper opening section 7, thus the air or water in the device is efficiently circulated, and the underground heat can be pumped.