To increase moduls of section and reduce cost for reinforcement and construction man-hour by providing a recessed water drain groove of which top face is opened on a floor face of a waterproof pan.

A waterproof pan 20 is formed into rectangular shape, and two recessed water drain grooves 22, 23 of which top face is opened in such a manner that they cross substantially at the center in the diagonal direction of a floor face 21 of the waterproof pan 20 are provided. Flow gradient for draining water is formed from one end side to other and side on the floor face 21, and a water drain groove 24 is provided at substantially the center of the floor face 21 on which water drain grooves 22, 23 cross. Consequently, its modulus of section is increased, the strength of the floor face 21 is increased for load which is applied from the above, and deflection is reduced greatly when compared with a flat floor face.