To solve the following problems: a conventional cylindrical heat exchanger extracts and dissipates geothermal heat at low efficiency, it is difficult to install it and costly; and the area within 2m below the surface of the ground is apt to be influenced by a change in outside air temperature, and the heat exchanger having the same shape from the bottom to the top is subjected to the above influence, resulting in low heat-collection/dissipation efficiency.

A heat exchanger employs unit cylindrical structures 29, 31, 35 having cylindrical outer tubes formed by curving a metal plate bent to form a polygonal shape, a wave shape, a fin shape or composite shape thereof. The heat exchanger has higher heat-collection/dissipation efficiency than a conventional cylindrical heat exchanger due to its larger surface area, and can be easily installed in crowded residential areas. Excellent heat-collection/dissipation property is ensured in a shallow depth due to the unit structure.