By causing a screw receiving portion to protrude from a cord fastening device side of a main frame and by forming the screw receiving portion in a cup-like stepped shape that is recessed from a lower-side chain cover side toward a lower surface side of the main frame, a thickness 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 of the main frame. Further, a depth of a concave portion of the screw receiving portion is formed to be a depth such that a distal end portion of a screw to be used does not protrude from the main frame.
