To avoid increase of labor and costs required in construction of a building of a reinforced concrete structure having a wing wall in which a structural slit is buried and a spandrel wall connected to it.

In order to form a structural slit (24) inside a wing wall (16) of a reinforced concrete structure building, after forming a floor slab, a mold form (26) for forming the wing wall and a mold form (28) for forming a spandrel wall (18) are installed on it, a structural slit material is horizontally supported inside the mold form of the wing wall with at least one of separators (34, 36) applied to both mold forms of the wing wall and the spandrel wall as support means and through a metal fitting (40), and thereafter, concrete is placed inside the mold forms of the wing wall and the spandrel wall.