To provide a structure of a pile head of a hollow existing pile that uses the hollow existing pile as an underground heat exchanger by arranging a pipe for a heat medium flow in the hollow existing pile as a foundation pile of the structure, and can secure appropriate strength and durability even if a guide pipe for passing the pipe for heat medium flow in foundation reinforced concrete interferes with the reinforcing bar.

In the foundation reinforced concrete 1, a steel flange 31 is attached to the guide pipe 7 in a place 20 where the reinforcing bar 19 interferes with the guide pipe 7, the reinforcing bar 19 is cut at the interference place 20 and arranged, and the cut and arranged reinforcing bar 19 is fixed to the steel flange 31.