Abstract: The air bag 3 according to this invention is furnished with exhaust outlet 6 to discharge internal gas, and the material 8 with and heat-shrinking property is provided, which blocks at least a part of the exhaust outlet 6 in normal case and shrinks to such direction as to increase the passage area of said exhaust outlet when reaction gas is introduced from the inflator 2. Therefore, the passage area of the exhaust outlet 6 is relatively narrowed down by the heat-shrinking material 8 in normal case. For this reason, it is rapidly expanded by reaction gas from the inflator 2. Thereafter, the material 8 is shrinked by the heat of reaction gas and the passage area of exhaust outlet 6 is increased. Thus, more reaction gas flows out of air bag 3, and this alleviates the impact when the air bag 3 receives the person in the vehicle.