To dispense with a concrete stop iron plate at a fastener part for fixing an exterior wall plate.

A floor backing material 1 is provided with cutout parts 3, and fasteners 4 are installed in the cutout parts 3. Exterior wall plates 2 are fixed to the building side through the fasteners 4. A flexible concrete antirunning material 5 is installed in a U-shape bent state in the outer edge position of the cutout part 3 and fixed to the floor backing material 1 using an adhesive tape or the like. A clearance G between the floor backing material 1 and exterior wall plate 2 is closed with a fire resistant layer material T. Concrete is then placed on the floor backing material 1.