To improve work efficiency by dispensing with a surface finishing process such as partial plating or partial coating after a process of connecting and fixing both ends of a pipe frame, and dispense with post-fitting a hinge metal or fixtures by a bolt or a small screw to reduce the number of parts.

A pipe is bent to form a substantially square frame-like seat frame 11. Pipe connecting members 15a, 15b are fitted to the opposite end parts 12a, 12b, 13a, 13b of pipe frames 12, 13, and the ends of the pipe frames 12, 13 are fixed by the pipe connecting members 15a, 15b. A fixture 17 is previously welded to the pipe connecting members 15a, 15b, and the fixture 17 is subjected to surface finishing 23 such as plating or coating. After that, both end parts of the pipe connecting members 15a, 15b are fixed to both end parts of the pipe frames 12, 13 by caulking 25.