To stably fix a supporting shaft to a pipe member while preventing rotation of the supporting shaft on a curved face and stably installing other members in the pipe member without causing any backlash and the like by fitting the supporting to a shaft side face of the pipe member via a curved face of a head part, so as to fix it in the pipe member.

A supporting shaft 3 is provided with a head part 3b in which a curved part, 3a matching with the shaft inside face of a vertical lever part 1a (1b) serves as an opposed contact face, and in the center position of the head part, 3b, a shaft part 3c raised from the curved face 3a is molded with the head part, 3b. Using the supporting shaft 3, a bracket plate 2 is fitted on the shaft outside face of the vertical lever part 1a (1b), and a clamping bolt 5 is inserted via a stopper hole 2b arranged on a plate face 2a in the bracket plate 2 so as to be screwed to a thread 3d in the supporting shaft 3, and as a result, the bracket plate 2 can be fastened to the vertical lever part 1a (1b).