A gas generator for an air bag capable of easily and unfailingly closing a gas discharging port formed in a housing. An amount of accommodated gas generating agent can be varied and the gas generating agent can be ignited and burnt unfailingly even by ignition devices having normal output. The gas generator has sealing structures as small as possible and a simple structure, and can be manufactured easily. Activation performance and activation timing can be adjusted at multiple stages. For instance, the air bag housing may be provided at its peripheral wall with a plurality of gas discharging ports, an ignition device to be actuated upon impact, and a gas generating agent to be ignited and burnt by the ignition device, wherein the gas discharging ports are arranged in the axial direction of the housing to form a gas discharging port line. Other configurations are also disclosed.