To provide an energy saving type underground heat exchanger installed easily and inexpensively in a home, capable of exhibiting concurrently two functions contributing to city environmental improvement, capable of making the best use of a natural phenomenon and a natural raw material, and integrated with a permeation well.

A heat exchanger and the permeation well are sharedly used in a bored hole of same large diameter drilled into the ground. An upper hole wall in the large-diametric bored hole is used as the permeation well, and a lower hole wall is used as a heat-collecting and heat radiating part of the heat exchanger. A water storage pipe is arranged over the in-hole full length, to store rain water and to make it flow. The rain water in the storage pipe is replaced naturally by the in-pipe flow generated by the supply of the rain water and the permeation of the rain water. A storage water tank is installed in an upper part of the large-diametric bored hole, to continue stable supply to a rain water lower part facility.