PURPOSE: To form a snap ring groove with the roll over on a drum opening end side being suppressed small, and without causing any fall or crack in a device where the snap ring groove is formed in the half blanking in the tooth tip part of the internal gear spline to be formed on the circumferential wall part of a clutch drum.

CONSTITUTION: A punch 9 to be bitten in the tooth tip part B of a clutch drum, and a work receiver 8 to be abutted on the outer circumferential surface of the clutch drum are provided. A clearance groove 8a to allow the outward extrusion of the chip accompanied by the biting of the punch 9 is formed in the work receiver 8. The clearance S between the groove wall surface on the drum opening end side of the clearance groove 8a and the punch 9 is ≤4% of the thickness of the tooth tip part B to reduce the roll over L on the drum opening end side of the snap ring groove C. The radius R of curvature of a corner part on the drum opening end side of the clearance groove 8a is ≥0.5mm so as to prevent the fall of the groove wall on the drum opening end side of the snap ring groove C and the generation of cracks.