To ensure sound-insulating performance without complicating constitution, and to prevent the generation of the dew condensation of the rear of base plywood by installing sound-absorbing materials and sound-insulating sheets on the rear among battens of the base plywood reinforced by a plurality of the battens in a laminating manner in order from the foundation plywood side and arranging the base plywood side to the side of an atmosphere susceptible of condensation.

Batten cleats 3 are mounted to both end sections and a central section in the cross direction of the rear of plywood 2 for interior finishing as base plywood along the longitudinal direction, and the plywood 2 for interior finishing is reinforced. Sound- absorbing materials 5 such as foamed polystyrene, formed polyethylene, etc., are set up among the batten 3 and fixed on the rear of the plywood 2 for interior finishing through vinyl acetate or epoxy resin adhesives 4, sound-insulating sheets 7 are fast stuck and fastened on the rears of the two sound-absorbing materials 5 through adhesives 6, and the plywood 2 for interior finishing is disposed towards the atmospheric side, in which dew condensation is easy to be generated. The sound-insulating sheet 7 manufactured by filling a resin material such as polyvinyl chloride, nylon, etc., with iron oxide and molding the resin material in a sheet shape is used, and the outer edge section of the sound-insulating sheet 7 is fast stuck and fixed onto the battens 3.