To suppress variations in quality by simplifying processing, since a structure of a conventional art in which a slit is formed at a rear end of a rear swing arm and an end-piece plate material is sandwiched therebetween to integrate the rear swing arm requires much effort in formation and is liable to cause variations in quality.

A rear edge of an arm 15 is press-molded into a near-crank shape to form a crank-shaped portion 30, an insertion space 33 formed between an outer vertical wall 31 and an inner vertical wall 32 is made narrower, and a rib-like upper wall 34 and a rib-like lower wall 35 are projected from the top and the bottom thereof. A tabular member is press-molded into an end-piece 16 in which an insertion portion 40 inserted into the insertion space 33, slits 47 and 48 fitted into the upper wall 34 and the lower wall 35, respectively, an upper arm 41 to contact with the top of the upper wall 34, and a lower arm 42 to contact with the lower wall 35 are integrally formed. The insertion portion 40 is inserted into the insertion space 33, and a joint between the end piece 16 and each rear edge of the outer vertical wall 31 and the inner vertical wall 32 is welded to each other to be integrated.