The head rest is detachably mounted to the seat back of the front seat and the air bag unit with an air bag mounted therein is mounted to the head rest. The air bag is expanded toward the passenger seated on the rear seat in response to an explosion signal from the control unit into which signals are entered from impact sensors. The air bag is so disposed as to expand only when the passenger is seated on the rear seat. The posture and position of the air bag unit is adjusted in association with the movement or displacement of the front seat to which the air bag is mounted, thereby improving the ability of the expanded air bag to confine the passenger seated on the rear seat and protecting the passenger in a safer fashion. Further, the air bag unit is mounted to the head rest of the front seat, so that the air bag can be exchanged with ease and at a cheap cost without needing to exchange the front seat itself.