PURPOSE: To form a poly V groove forming part in the specified thickness and to facilitate the high accuracy forming of a V groove by holding both ends in the axial direction of the bulging part of the preformed cup-shaped blank bulging a cup shaped blank peripheral wall to the external side and pressurizing it from outside.

CONSTITUTION: A preformed cup-shaped blank 1b is supported by being fitted to a lower part rotating inner die 21 and the release end in the axial direction of the blank 1b is held by abutting to a die face 21a. A slightly thickening rough V groove forming part 11 is formed at the peripheral wall bulging part 5 of the blank 1b with the regulated holding of the end part in the axial direction by the die faces 23b, 23c of a rotary forming roll die 23 and the pressurizing force by the die face 23a and a rough thickening cup shaped blank 1e is obtd. This blank 1e is supported by being fitted to a lower part rotating inner die 24 and the release end in the axial direction of the blank 1e is held by abutting it to a die face 24a. Under the follow regulation by an upper part rotating outer die 22 and rotary forming roll 23 it is pressurized by a rotary forming roll die 25, the rough poly V groove forming part 11 of the blank 1e is thickened by a die face 25a to form a poly V groove forming part 12, rough poly V grooves 7b, 7c are simultaneously formed and a thickened cup shaped blank 1f is obtd.