To store the rain water capable of being used as water for miscellaneous use or emergency water by allowing the rain water to quickly percolate the ground when a large quantity of rain exceeding the draining capability of a sewage conduit or a river falls.

A lower drain port 12b is formed on the bottom plate 12a of a pit main body 12 covered with a cover 18 on the upper face, and rain water 15 is guided to the pit main body 12 from a rain water guide port 12. An overflow hole 12f is formed at the upper section of the side plate 12c of the pit main body 12, and an upper discharge port 12e is formed at the position lower than the overflow hole 12f by the prescribed distance on the side plate 12c of the pit main body 12. A ring-like pan 19 is inserted into the pit main body 12 to block the upper drain port 12e. The lower drain port 12b is openably blocked by a plug 14, and a float 16 having the specific gravity less than 1 is stored in the pit main body 12. The plug 14 and the float 16 are connected by a linear member 17, the linear member 17 is strained and the float 16 is lifted by buoyancy when the level of the rain water 15 reaches the vicinity of the lower face of the pan 19, and the rain water 15 is discharged from the lower drain port 12b.