To prevent the leakage of water to a lower floor even when the leakage occurs on an upper floor by forming a grade on a base floor, laying a laying member and forming a drain hole to a side wall while being oppositely faced to an end section on the lower side of the laying member.

Laying members consisting of vinyl sheetings 16, 16, etc., and having water impermeability and flexibility are laid on base floors 13, 13 partitioning upper and lower floors while forming grades. Side walls such as upright walls 14... are disposed upright at the upper edge sections of the base floors 13, and drain holes 15... are formed on the side walls 14 while being oppositely faced to end sections on the lower sides of the laying members 16. Floor boards 21,... are laid on the top faces of the laying members 16 through support legs such as bundles 17.... Accordingly, even when the leakage of water occurs on the upper floor, leaking water is recovered by the laying members 16 and discharged from the drain holes 15, and leakage to the lower floor can be prevented.