An air bag device includes an air bag which is accommodated in a folded state along sides of a seat back and a seat cushion of a seat for an occupant. Upon side collision of a vehicle, the air bag is expanded by a gas generated by an inflator and deployed between an inner surface of a side of a vehicle compartment and an occupant. The air bag is disposed outside a lap belt of a seat belt device, and hence is deployed from the sides of the seat back and the seat cushion, so that a shock-absorbing effect can be effectively exhibited.