To provide a seat back which can avoid the fracture and opening, vertically larger than necessary, of a part to be fractured in the expansion and development of a side airbag in the collision with a vehicle, can suppress the delay of arrival time of the side airbag to a seated person, due to the fracture and opening of the part to be fractured, can reduce man-hour, and can simplify the management of sewing thread.

The seat back includes: a sewing part 5, formed by sewing a pair of terminals 11A, 12A of a skin material with each other, on the side surface; the part 6 to be fractured in the sewing part 5; and the built-in side airbag to be expanded and developed by fracturing the part 6 to be fractured. The end part of the part 6 to be fractured in the longitudinal direction is sewn so that a projected piece 13 formed in one terminal 11A of a pair of the terminals 11A, 12A of the skin material winds another terminal 12A.