To form a yoke part at the end of an arm part integrally at low cost.

The arm part 2 is formed by cutting a light alloy to a prescribed dimension by extrusion-molding so as to be formed in a roughly I-shape in cross section of a first surface 16 and a second surface 17 parallel with each other and a third surface 18 connected to these first and second surfaces crossingly to each other. One end part of the third surface 18 at the longitudinal end of the arm part 2 is cut off to form a cutout 19. The end parts 16a and 17a of the first and second surfaces 16 and 17 are separated from each other and spread to form a yoke part 3 formed in a fork shape of roughly U-shape and a cylindrical bush 4 is supported between the forks of thee yoke part.