Abstract: In a die forging process, projections (4a) are formed on an inner circumference thereof. The projections (4a) are inserted into holes (46) provided in a disc plate (45). The projections (4a) are crushed by a punch 49 so that the disc plate (45) is securely fixed on an inner circumference of the annular gear 41. The process can eliminate a cutting and an electric welding to shorten a manufacturing time.