PURPOSE: To facilitate the mounting of a floor panel to a beam and the mounting of a waterproof sheet on the floor panel by providing waterproof setting bed members on the surface of the floor panel in the floor panel of a composite board and mounting two opposite sides respectively to beams of a steel framed structural skeleton with bolts.

CONSTITUTION: A floor panel 11 is so constituted that a cement board 12, a thermal insulating material, a cement board 14 and a calcium silicate board 15 are laminated subsequently from the lower part of a floor, and a channel member 16 is provided to the peripheral edge. After that, angle members 17 are mounted respectively to two side heading surfaces to the panel 11, and circular waterproof sheet fixing waterproof setting bed members 19 are mounted to specific positions on the upper surface of the calcium slilicate board 15. Then, the angle members 17 provided to two sides opposed to the panel 11 are mounted on the upper surface of skeleton beams 21, and the angle members 17 are interlocked with the beams 21 through bolts 22. One waterproof sheet 20 is laminated on the upper surface of a plurality of panels 11 to be arranged, parts corresponding to the setting bed members 19 are heated, and the sheet 20 is welded to the setting bed members 19 and is mounted.