A body member and flange members are butted together at first and second abutment portions, and are retained by first and second backing jigs. In this arrangement, inner circumferential surfaces of the first and second abutment portions are fitted on and in close contact with outer circumferential surfaces of the first and second backing jigs. Subsequently, the friction stir welding is performed along each of the first abutment portion and the second abutment portion by using a probe which is rotated at a high speed.