In an upper frame member of seat back framework, an upward-slippage prevention portion is simply defined to prevent a seat occupant's back portion from being slipped or dislocated on and along a seat back in the case of rear-end collision. The upward-slippage prevention portion has a sloped region for receiving the seat occupant's back portion which is to be slipped or dislocated on and along the seat back under inertia when a rear-end collision occurs and thereby preventing an undesired upward slippage or dislocation of that seat occupant's back portion which will result in damage of his or her neck or cervical vertebrae.