PURPOSE: To prevent the products from being flawed and to prevent the die from being weared by clamping the U-letter like base material to be worked with a pair of die components and pressurizing the flange part of the base material to be worked with the punch to the inner side.

CONSTITUTION: U-letter like base material 1 to be worked which is deformed with the spring back is mounted on the die 14, when the upper die stand 2 is driven downward, the elastic body 3 is deformed, and the base material 1 to be worked is clamped with the pad 4 and the die 14. At the same time, the projecting chamfer part 12 of the coining punch 7 comes in contact with the chamfer part 16a of the fixing cam 16, the pressurizing force is applied toward the inner side to the coining punch 7, and the coining punch 7 is moved horizontally against the elastic body 10. And the flange part 1a of the base material 1 to be worked is pressurized with the coining punch 7, and bent with the highly rough degree. In this time, because the stepped part 11 in contact with the flange top end of the base material to be worked is set on the coining punch 7, the flawing for the products and the wear for the die can be prevented.