PURPOSE: To improve the pitch accuracy of a groove and to reduce a forming cost by arranging the die forming the groove forming teeth of specific pitches respectively on the inner peripheral face and outer peripheral face and a punch, and subjecting a cup like blank to push-forming into a die.

CONSTITUTION: A cylindrical die 10 having groove forming teeth 11 on the inner peripheral face is provided and a circular columnar punch 12 having the teeth 13 for groove forming is arranged on the outer peripheral face as well. In this case, the teeth 11 of the die 10 are formed in equal pitches according to the number of grooves to be formed on the external case as a product and an upper end 11a is formed in the inclined face inclining in specific down slope. The external case blank stock 15 formed in cup shape in advance is set inside the die 10 and after arranging a counter punch 14 the punch 12 is descended to form a groove 16 on the blank 15. Due to the groove 16 being formed on the blank 15 with one punching the pitch accuracy of the groove 16 is improved to reduce the forming cost.