PURPOSE: To simplify an operation process after concrete placing as well as to improve the extent of operation efficiency by setting up a vertical supporting plate, arranging a square pillared ledge along the predetermined position and, after concrete is placed, removing this ledge away and installing a level difference there.

CONSTITUTION: A vertical supporting plate 11 is set up on the bottom surface of a slab form 6, and plural pieces of support members 12 are properly attached tight, thereby supporting the square pillared ledge S with the vertical supporting plate 11 and this support plates 12. Next, concrete is placed there, and then the vertical supporting plate 11 and the support member 12 both are embedded in the concrete. In assuccesion, prior to the concrete hardened, the ledge S is removed away, and mortar is padded in the spot removed with the ledge S at need and thus compensation is carried out. When the concrete is fully hardened, a level difference is thus completed. With this constitution, the vertical supporting plate 11 and the support member 12 are left embedded intact, thereby dispensing with their removal in this way., so that a span of opening time is reducible.