To easily attach/detach a forming piece to/from a slab mold body by attaching and fixing a plurality of mold fixing parts projected downward of a pedestal having a female screw part in the center to the slab mold through a bottom part of the forming piece body, and placing concrete to form a through hole.

A female screw part 2 is provided on a center part of a pedestal 1 formed circular by a thin steel plate, and an appropriate part of the pedestal 1 is notched and folded downward to form a bending mold fixing part 3. A through hole 6 of a bottom part of a horn-shaped forming piece body 4 is aligned with the position at which the through hole is provided on the slab mold A, the pedestal 1 is placed in the forming piece body 4, and the mold fixing part 3 is attached and fixed to the slab mold A through a fitting hole 7. A lid member 10 is attached to an upper end of the forming piece body 4, the concrete is placed, and the lid body 10 is removed after the curing, a male screw part of a drawing tool is screwed into a female screw part 2 of the pedestal 1, the forming piece body 4 is removed after removing the pedestal 1, and a through hole is formed. The forming piece can be easily attached/detached to/from the slab through hole, and the working efficiency is greatly improved.