An upper end portion of a seat backrest and a lower end portion of a head rest are superposed together. A rear end portion of an upper portion cushion member and a lower end portion of the seat backrest are superposed together. These superposed portions are sewn together by thin threads which break when pressing force of a predetermined value or greater is applied to the seat backrest. Thus, when pressing force of the predetermined value or greater is applied to the seat backrest, the thin threads break. A portion of the seat backrest which a shoulder portion of a seated occupant contacts, and a portion of the seat backrest which buttocks of the seated occupant contact, are enlarged, and impact force can be damped.