An air bag 1 is provided. At a vehicle side impact, the air bag 1 unfolds from an upper part of a vehicle body downwardly. The air bag 1 includes an airbag body 10 having an inside foundation cloth 8a and an outside foundation cloth 8b, and a pair of inner-cloths 19a, 19b arranged in the airbag body 10. The inside inner-cloth 19a is joined to the outside inner-cloth 19b through two inner junctions 21, 21 separated from each other in a fore-and-aft direction of the vehicle. The inner-cloths 19a, 19b have front ends, rear ends and intermediate portions between the inner junctions 21, 21, all of which are joined to the foundation cloths 8a, 8b through overlapping junctions 22a, 22b. In arrangement, the overlapping junctions 22a of the inside inner-cloth 19a are shifted from the overlapping junctions 22b of the outside inner-cloth 19b forwardly. Consequently, triple-layer tubular expansion chambers 25a, 26, 25b are defined in the airbag body 10, allowing a passenger's head to be received certainly.