A gas generator comprising: a housing including a top plate (11 a), a bottom plate and a circumferential wall (11 b, 12b) provided with a gas discharge port; an inner cylindrical member disposed inside the housing and defining an ignition chamber therein; a space outside the inner cylindrical member in the housing being axially divided into two spaces by a partition member (60, 65); a first combustion chamber charged with a first gas generating agent and being formed in one of the two spaces, the first combustion chamber being in communication with the ignition chamber by a first communication nozzle provided in the inner cylindrical member; a second combustion chamber charged with a second gas generating agent and being formed in the other of the two spaces, the second combustion chamber being in communication with the ignition chamber by a second communication nozzle provided in the inner cylindrical member; characterized by a plenum chamber defined by the partition member (60, 65) so as to axially divide the space outside the inner cylindrical member in the housing into the first combustion chamber and the second combustion chamber, an inner side of the plenum chamber facing the inner cylindrical member, an outer side of the plenum chamber facing the circumferential wall (11b, 12b) provided with the gas discharge port, the first combustion chamber being in communication with the plenum chamber, and the second combustion chamber being in communication with the plenum chamber, an axial position of the first communication nozzle (80, 180) being adjusted to be farther away from the plenum chamber, than an axial position of the second communication nozzle (82, 182), during actuation, the first gas generating agent located in the first combustion chamber to be burnt up earlier than the second gas generating agent located in the second combustion chamber. a housing including a top plate (11 a), a bottom plate and a circumferential wall (11 b, 12b) provided with a gas discharge port; an inner cylindrical member disposed inside the housing and defining an ignition chamber therein; a space outside the inner cylindrical member in the housing being axially divided into two spaces by a partition member (60, 65); a first combustion chamber charged with a first gas generating agent and being formed in one of the two spaces, the first combustion chamber being in communication with the ignition chamber by a first communication nozzle provided in the inner cylindrical member; a second combustion chamber charged with a second gas generating agent and being formed in the other of the two spaces, the second combustion chamber being in communication with the ignition chamber by a second communication nozzle provided in the inner cylindrical member; characterized by a plenum chamber defined by the partition member (60, 65) so as to axially divide the space outside the inner cylindrical member in the housing into the first combustion chamber and the second combustion chamber, an inner side of the plenum chamber facing the inner cylindrical member, an outer side of the plenum chamber facing the circumferential wall (11b, 12b) provided with the gas discharge port, the first combustion chamber being in communication with the plenum chamber, and the second combustion chamber being in communication with the plenum chamber, an axial position of the first communication nozzle (80, 180) being adjusted to be farther away from the plenum chamber, than an axial position of the second communication nozzle (82, 182), during actuation, the first gas generating agent located in the first combustion chamber to be burnt up earlier than the second gas generating agent located in the second combustion chamber.