Abstract: After the pressure of the gas in a first chamber reaches the maximum value, the cover for the air bag is broken up. The cover is broken at the pressure of the maximum value, and there is almost no waste of the gas energy. Thus, the cover can be broken up by the small amount of the gas energy and the heat losses are extremely reduced, so that the amount of the gas generating agents are made small. Also, the cover is broken by the burning of the first chamber and the air bag is inflated at the first stage by expansion and inertia of the gas. There is provided a mechanical timing for ignition delay, and then the bag is fully inflated by the burning of a second chamber. Therefore, the bag can be desirably inflated.