Abstract: The present invention provides a gas generator capable of easily and unfailingly closing a gas discharging port formed in a housing. The invention provides a gas generator for an air bag in which an amount of accommodated gas generating agent can be varied and the gas generating agent can be ignited and burnt unfailingly even by the ignition means having normal output. The invention provides a gas generator which has sealing structures as little as possible and in addition to a simple structure and can be manufactured easily, and in that a activation performance and activation timing can be adjusted at a multiple stage.