A seat back of vehicle seat, on which a headrest unmovable in fore-and-aft direction is mounted, is constructed such that a resilient support member for supporting a back portion of seat occupant is provided therein, and the upper region of that resilient support member is operatively connected, via movable connecting element, with a basing element. When a rear-end collision occurs, the resilient support member is displaced far backwardly by excessive great load applied thereto from the seat occupant, overcoming a biasing force of the biasing element, thereby causing the basing element to stretch long, so that the resilient support member with the seat occupant's back portion thereon is displaced backwardly and upwardly under guidance of the movable connecting element so as to quickly bring a head of the seat occupant to a point close to the headrest, prior to the head being directly pressed against the headrest.