Since an inside piece of a joint block is abutted to a rib of a second opposed piece under a pressure by engaging a first opposed piece of a frame member with the joint block, the inside piece can be securely abutted at the position of the rib even when the second opposed piece and the joint block are slightly deformed, which secures a predetermined joint intensity by preventing a variation of the abutment position. Since the joint block is abutted with the rib, the pressure per a dimension can be more increased than abutting with the whole surface of the inside piece, which consequently increases the frictional force to improve the joint intensity.