To provide an underground temperature stratified heat accumulating water tank with inexpensive manufacturing costs and high heat accumulating property.

In this underground temperature stratified heat accumulating water tank, a rotary pressure insertion steel pipe pile is rotary pressure inserted and buried in the ground as a base pile supporting a building. A bottom cover is installed at the tip or the middle of the rotary pressure insertion steel pipe pile to seal the pile for storing water. A water filling piping and an intake piping are installed inside the rotary pressure insertion steel pipe pile.