PURPOSE: To enable adjustment of the height and the level of a floor wiring fixture supporting board by arranging a left screw adjusting screw provided with a driver stop groove and an axial screw hole in the upper endface, adjustably in vertical direction, on a box burried in the floor.

CONSTITUTION: Level of a box 1 is adjusted through a screw 4 before it it burried in a concrete floor F. When a floor wiring fixture supporting board 6 is fixed on the open upper face of the box 1, left screw receivers 7 are arranged at intermediate height positions at four corners of the box 1 then left screw adjusting screws 8 are fixed thereto thus making the supporting board 6 adjustable vertically. A slot 9 for screwdriver and an axially penetrating right screw hole 10 are made in the upper end face of the screw 8. The floor wiring fixture supporting board 6 is secured onto the adjusting screw 8 through a screw 12 and adjusted vertically through the stop groove 9. Since the adjusting screw 8 is threaded leftward, it is not loosened through the right screw 12 thus enabling efficient adjusting/fixing.