A center-foldable seat back has a seat back upper part that can be tilted forward relative to a seat back lower part. A trim cover is provided, sewn along the entire periphery at the mutual contact position of an upper and lower cover, the pulled-in state is modulated by cloth pieces, thereby preventing wrinkles. The seat back is provided with the trim cover in which one cloth piece one side edge of which is sewn to the ends of the lower and upper cover is provided at least in the central portion on the inside of the trim cover, the cloth piece is extended inward, the other side edge thereof is sewn to a fastener, the central portion on the back side is extended inward and sewn to the fastener, and the central portion on the inside of the trim cover is sewn to open and close by one fastener.