An airbag has a first inner panel and a second inner panel arranged substantially in a concentric relation with a front panel and a rear panel in an interior of an airbag, and outer peripheral edge portions are stitched to each other by a high-strength seam. A center portion of the first inner panel is stitched to a center portion of the front panel by a seam. A center portion of the second inner panel is connected to a center portion of the rear panel by a retaining ring. The second inner panel is formed with a communication port between a first chamber and a second chamber. Midsections of the inner panels between the outer peripheral edge portions and the center portions are connected by a tear seam. The tear seam is adapted to be broken when the pressure in the first chamber reaches a predetermined pressure.