Abstract: In a seat back structure, an auxiliary frame having a headrest mounted thereon is provided independently of a seat back frame and movable via link element and guide element along a predetermined path with respect to the seat back frame. The guide element includes: a bracket having one end pivoted to the seat back frame; a guide hole formed in the bracket; and a guide pin provided to the seat back frame, the guide pin being slidably engaged in the guide hole. A buffer member is provided to another end of the bracket. Shortly after the headrest has been moved to a headrest receiving point in a rear-end collision case, the headrest is subjected to abrupt backward displacement under backward inertia of seat occupant, but, such abrupt backward displacement of headrest is decelerated by the buffer member.