Abstract: An air bag having inflatable ribs is provided for use in connection with a gas generator for protecting a passenger from injury due to impacts. A gas generator includes a housing, a gas generation device disposed inside the housing, an air bag fitted to the housing, and a gas jet opening. The air bag includes an air storage chamber for receiving air via a plurality of air intake ports disposed in the air bag and storing it therein. Also included are a plurality of hollow ribs each being disposed in the air storage chamber, each having one end thereof in communication with the gas jet opening. Each of the hollow ribs comprises a tube-like gas storage chamber for receiving a combustion gas via the gas jet opening and storing it therein. The gas storage chamber is equipped with an opening for supplying the combustion gas inside the gas storage chamber to the air storage chamber.