To form a fine concrete surface in a joint of a foundation form.

The forming member 10 is a metal-made or a resin-made bar- shaped body, and it includes a base section 11 and a thin plate section 13. The base section 11 is so constituted that a right angled section in the longitudinal direction is formed in an isosceles trapezoidal-shape, and a long side thereof becomes a flat contact surface 12 coming into contact with the foundation form 20. The thin plate section is a thin plate having 1 to 2 mm in thickness, and it is vertically adhered and fixed to an intermediate position in the direction of the width of the contact surface. The thin plate section of the forming member is inserted between vertical frame plates of each of the adjoining foundation forms, and the contact surface of the base section is placed in a state to adhere it to the surface of a molding plate of the foundation form. A rod 27 of a fixed member 26 is inserted into a mounting hole of the vertical frame plate and an insertion-hole 13a of the thin plate section of the forming member, both vertical frame plate are fixed by locking a U-shaped locking section 28 with both vertical frame plate, and the forming member is held between the foundation forms.