To utilize a foundation pile of a non-solidified pile head structure in safety as an underground heat exchanger, and to simplify maintenance of heat medium piping.

In this constitution of the pile head portion of a hollow ready-made pile utilizing the hollow ready-made pile as the underground heat exchanger by placing piping for circulating heat medium in the hollow ready-made pile as a foundation pile of a structure, a pile head supporting member 7 is mounted between the foundation concrete 5 of the structure and a pile head portion of a cylindrical ready-made pile 1 to support the foundation concrete 5, the pile head supporting member 7 has a square steel pipe 9 laterally placed, and a short circular steel pipe 11 connected with a lower face side of the square steel pipe 9 and inserted into the pile head portion of the cylindrical ready-made pile, and a communication hole 13 for inserting the piping 3 for circulating the head medium is formed on a connecting portion of the square steel pipe 9 and the circular steel pipe 11.