By causing a screw receiving portion 20e to protrude from a cord fastening device 150 side of a main frame 20 and by forming the screw receiving portion 20e in a cup-like stepped shape that is recessed from a lower-side chain cover 30 side toward a lower surface side of the main frame 20 by press working or the like of a sheet metal material, a thickness (H-L) of a bottom portion of the cup shape in which a screw hole is formed can be formed to be equal to a plate thickness t of the main frame 20, and a required screw hole depth can be uniformly and accurately secured while maintaining a required minimum plate thickness. Further, a depth L of a concave portion of the screw receiving portion 20e is formed to be a depth such that a distal end portion of a screw 130 to be used does not protrude from the main frame 20, and thus interference between the distal end portion of the screw 130 and the lower-side chain cover 30 can be surely avoided.
