The present invention provides a gas generator, relating to a gas generator for a side air bag or the like, in which the gas purification and cooling efficiency in a filter can be enhanced, the filter is not an obstacle in the release of the gas and the structure is simple. A gas generator for an airbag, in which an inner cylindrical member is stored in a housing, wherein the inner cylindrical member has a second outside diameter portion abutting against the inner peripheral surface of the housing, and a first outside diameter portion that is smaller in outside diameter than the second outside diameter portion, the tip end of the first outside diameter portion side abuts against the closed end portion (one end portion) of the housing, the filter is provided on the outside of the inner cylindrical member in the radial direction of the first outside diameter portion, and the ignition means is provided on the opposite end portion (the other end) from the closed end portion of the housing.