A nut anti-slipping member is provided, which has a nut insertion groove and covers bottom end portions of one-side opening end of a nut insertion hole. A nut is inserted in the nut insertion hole with the nut deviated from the nut anti-slipping member. In case when the nut is of a square-pole-shape type with rectangular section, the entire nut can pass upper parts of the anti-slipping member when the nut is inserted in the nut insertion hole in a horizontally longitudinal position, and a bottom part of the nut can pass the nut insertion groove when the nut is inserted in the nut insertion hole in a vertically longitudinal position.