A gas generator includes a housing and a gastight container which accommodates a gas generating agent. The gastight container includes a cup body and a cover body. The cover body includes a bottom portion and a fold-over portion which covers an opening end of the cup body. The cover body is fixed to the cup body by providing, in the fold-over portion in a portion covering an outer circumferential surface of the opening end of the cup body, a first swaging portion in a portion of the fold-over portion which corresponds to a sidewall portion of the cup body and defines a gas generating agent accommodation chamber at a position adjacent to the bottom portion and a second swaging portion in a portion opposed to a portion of the fold-over portion which covers an inner circumferential surface of the opening end of the cup body.