To make the strength high and make the weight small to facilitate assembling, transportation, and storage, to make one kind of mold sufficient when producing a pool combining only corner cells and make two kinds of molds sufficient when producing a pool combining corner cells and intermediate cells, to enable recycling by easy crushing, and to enable the avoidance of an impact by elasticity even if a strong impact is applied to the inside and outside of a pool.

In a built-up pool 1 in which a plurality of plastic segments 1a, 1b are connected by bolts 2 and nuts 3, respective segments 1a, 1b are formed by connecting a pair of cells 10a, 10a (10b, 10b) of the same shapes having a peripheral walls 10 and bottoms 11. Each of the cells has a double structure having a hollow part 12 formed of a polyethylene resin. A reinforcement means 13 is formed by partially integrating the peripheral wall 10 of the hollow part 12. A reinforcement means 14 is formed which partially connects the floor plate 11a and earth plate 11b of the bottom 11 of the hollow part 12.