To realize simplification and efficiency of installing-removing work of a duckboard-like floorboard.

The floorboard 12 has a recessed part 17 on the joining edge. This installing metal fitting 1 is composed of a cylindrical part 3 and a locking piece 6 arranged on the upper surface. The installing metal fitting 1 for inserting a machine screw 24 is loosely installed on a floor backing material 20 according to a joint position (a). A plate material 12 is arranged on the floor backing material 20 with the installing metal fitting 1 as a guide. The locking piece 6 is arranged along a joint (b). The locking piece 6 is rotated, and is positioned in the recessed part 17, and when fastening the machine screw 24, a projecting part 10 of an under surface of the locking piece 6 presses an under surface 18 of a recessed part wall, and the floorboard 12 can be fixed to the floor backing material 20 (c). When removing the floorboard 12, when rotating the locking piece 6 by loosening the machine screw 24 of both side installing metal fittings 1, the floorboard can be easily removed while holding a position of the other floorboard 12.