PURPOSE: To simplify structure and enhance safety by fixing the lower end of a vertical board, which is provided with a bracing notched area on the upper end, with the central area of a base board and fixing a reinforcing board with both sides of he vertical board and the base board.

CONSTITUTION: The lower end of a vertical board 12 provided with a bracing notched area 11 on the upper end is welded and fixed with the center of a square-shaped base board 10 where a reinforcing board 13 formed in the shape of a right angled triangle with a cut head is welded and fixed with both sides on the center of the vertical board 12 and the base board 10. Then, a spacer is placed on permanent concrete 2 with a specified spacing. A lower reinforcing bar 14 is fitted into the notched area 11 where a middle reinforcing bar 15 and an upper reinforcing bar 16 are placed thereon so that the reinforcing bars may be placed into a mat slab 1.