A plate is continuously press-bent, into a shaped member having a recess opening at one surface (i.e., lower surface). Then, the plate is bent back by pressing, at the front and rear ends, covering the openings of the recess and a side frame having open cross sections, each with a gap, is thereby formed. In the forming step, the side frame is not welded and has open cross sections, each with a gap. The side frames are then subjected to the next step (i.e., step of assembling the seat frame). In the step of assembling the seat frame, the side frames and coupling members are welded together, constituting a substantially rectangular seat frame. As the side frames are so welded in step of assembling the seat frame, a rigid structure each having closed cross sections with no gaps is formed.