PURPOSE: To make it possible to automatically constitute a waterproof floor by laying a waterproof sheet on the surface of a floor panel of a composite board and bolting two opposite sides respectively to beams of a steel frame structural skeleton to mount.

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, waterproof setting bed members 19 are mounted to the peripheral edge of the upper surface of the calcium silicate board 15, and the setting bed members 19 are heated and melted to lay a waterproof sheet 20. Then, angle members 17 provided to two sides opposed to the panel 11 are mounted on the upper surfaces of skeleton beans 21, and the angle members 17 are interlocked with beams 21 through bolts to mount the panel 11 to the beams 21. A joint waterproof sheet is stuck on a joint 25 between a plurality of panels 11 laid in parallel with the beams 21 to waterproof the joint 25. According to the constitution, the panel 11 is laid on the beams 21, so that a waterproof floor can be automatically constituted.