A gas generator includes a housing constructed by combining and joining a plurality of shell members. One of the plurality of shell members includes a cylindrical portion and a flange portion. The cylindrical portion is provided with a plurality of gas discharge openings including gas discharge openings different in opening area from one another. The flange portion is shaped such that a distance from an axial line of the cylindrical portion to an outer edge of the flange portion is non-uniform. When a perpendicular line is drawn to the axial line from a maximum outer geometry position in the outer edge of the flange portion most distant from the axial line, a gas discharge opening arranged closest to the perpendicular line is a gas discharge opening other than a gas discharge opening largest in opening area among the plurality of gas discharge openings.