To provide an inexpensive geothermal system that is easy to be assembled to a hous including those in an area with unfavorable weather conditions.

A pile 5 is arranged to penetrate a foundation slab 3a in mat foundations 3a, 3b, and an air conditioner 13 is installed. Further, a space 11 in the foundation has a structure where closed spaces are eliminated in each room by a foundation beam 3b, and air can be circulated from the inside of a pile and the space 11 in the foundation to an attic space 12 by blowers 8, 10 and 11, and ducts 9, 12. In a highly air-tight heat insulating building, the foundation slab 3a and the foundation beam 3b facing the outside are applied with heat insulating treatment such as foamed polyurethane foam.