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

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. Water filling piping and intake piping are installed inside the rotary pressure insertion steel pipe pile. A protruding object is mounted on an inner face a bottom cover mounting part of the rotary pressure insertion steel pipe pile or a steel pipe-made water tank in advance. A disc-like drop cover is placed to internally contact the steel pipe pile or the steel pipe-made water tank after burying and mounting the steel pipe pile or the steel pipe-made water tank. The periphery of the drop cover is fixed and the bottom cover is formed.

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