When a load is generated in an area of seatback that supports the pelvis of the occupant at the time of vehicle collision, a trigger mechanism located at a lower support member is activated. At this time, a displacement mechanism of an upper support member becomes available for activation. Since the upper support member moves in a rearward direction, the load applied to the thoracic vertebrae of the occupant can be kept small. Accordingly, the spinal shape of the occupant can be maintained. Additionally, the vehicle seat can mitigate the impact at the time of vehicle collision.