A fastening body having a flat substrate made of synthetic resin. The flat substrate has two synthetic resin layers having different molding shrinkage rates. With the two synthetic resin layers, the flat substrate can have a desired curvature in width direction. The fastening body is fixed by engagement in each of engaging groove portions of a mounting body. The mounting body has a substantially U-shaped section, formed along both side edges in a width direction, so that openings formed by the U-shaped section oppose each other.