Abstract: A sewing section for integrally sewing three members (a front cover member 23 covering a front surface of a seat back, a side cover member 24 covering a side of the seat back, and a ball edge 27 clamped between both the cover members 23 and 24) is comprised of a final sewing portion 26.sub.2 for integrally sewing the front and side cover members 23 and 24 and the ball edge 27 at a location nearer a body portion 27.sub.1 of the ball edge 27, and a temporary sewing portion 26.sub.1 for integrally sewing the front cover member 23 and the ball edge 27 at a location farther from the body portion 27.sub.1 of the ball edge 27. When an air bag is expanded, the final sewing portion 26.sub.2 is broken, and the ball edge 27 is separated from the side cover member 24 and moved in unison with the front cover member 23. Thus, when the air bag used upon side collision of a vehicle and mounted within a seat is deployed, the ball edge in the sewing section 26 is prevented from interfering with the air bag.