A gas generator for an air bag includes a housing having a gas discharge port, a first combustion chamber and a second combustion chamber, each of the combustion chambers being provided with a gas generating agent and an ignition device, being in the housing, the second combustion chamber being separated from the first combustion chamber by a cup member disposed within the housing, the second gas generating agent and the second ignition device being disposed within the cup member, the cup member being moved upon activation of the second ignition device, thereby forming a discharge passage for a gas generated in the second combustion chamber.