PURPOSE: To facilitate construction work, at the same time, require no strict accuracy for level adjustment of the arrangement surface and to effectively prevent the occurrence of an unnecessary space easy to occur in a butt section of drainage members adjacent to the longitudinal direction.

CONSTITUTION: A drain gutter is provided to the central part in the direction of the width of drainage members 1 formed of synthetic resin foamed particles molding bodies obtained by an in-mold molding method so that a linked space is formed between foamed particles, and surfaces having the drain gutter are opposed to each other to overlap a plurality of drainage members 1, 1, 1,... in both up and down stages so as to form a hollow channel inside the drain gutter and, at the same time, when they are linked in the longitudinal direction, interlocking projections 4 and interlocking recesses 5 are provided to the upper surface of both ends in the direction of the width of the drainage members 1 so that one drainage member 1 of one stage is made to lie across two drainage members 1 and 1 of the other stage to make it possible to combine them alternately with both up and down stages.