An air bag inflator has a cylindrical housing defining an internal space therein and having a convex portion protruding towards the internal space, and a cup member provided inside the cylindrical housing and having a cylindrical wall, provided with a vent hole that communicates inside the cup member with a space defined by the cup member and an inner surface of the cylindrical housing, and an end wall closing a first end of the cylindrical wall. The cup member is supported inside the cylindrical housing by making contact with the convex portion. An inflator assembly method includes, forming the convex portion, providing the cup member, and inserting the cup member inside the internal space of the cylindrical housing, such that the cup member abuts the convex portion.