A vehicle seat is mounted on a floor in a laterally inwardly moveable manner, and is accelerated by a power actuator inwardly upon detection of a vehicle side crash. The vehicle occupant is firmly held to the seat. By thus appropriately controlling the time history of the lateral acceleration of the vehicle occupant at the time of a side crash, the maximum acceleration acting on the vehicle occupant can be controlled. The inward movement of the seat provides a wider space between the vehicle occupant and door panel, and this contributes to the prevention of the injury to the vehicle occupant owing to the intrusion of the door panel into the passenger compartment at the time of a side crash.