an air bag gas generator has a cylindrical member that permits reliable welding of the cylindrical member with an inner surface of a ceiling portion of a housing even if the welding surface is slightly misaligned . it furthermore permits welding in such a manner that unsteadiness does not occur in the area where the inner surface of the ceiling portion of the housing and the inner cylindrical member abuts . in the air bag gas generator , which includes the inner cylindrical member in the housing , the inner cylindrical member has a peripheral wall portion bent to form a flattened surface portion as an extension of the one of that cylindrical member where it joins the inner surface of the ceiling portion .