To install a heat exchanger in a casing inserted to reach a water vein with high constructability, to prevent heat loss in an outside air influence range from a surface of the ground, and to allow efficient heat exchange with underground water heat.

The heat exchanger 2 inserted in the casing 1 inserted to reach a water vein M is composed by including a first unit 2-1 arranged on the bottom side of the casing 1, a plurality of second units 2-2 arranged in the casing 1 in line with the insertion depth of the casing 1, and a third unit 2-3 arranged in the casing 1 in the range of the outside air influence depth L. The first and second units 2-1 and 2-2 are provided with upper and lower return route-side tanks 5 and 6, return route-side long thin tubes 7, connection flanges 8, approach route-side long tubes 9 and 9a, and return route-side short tubes 10. The third tank 2-3 is provided with a connection flange 8, approach route-side long tubes 9b and 9c and return route-side long tubes 15 and 15a.