PURPOSE: To prevent occurrences of connection failure by putting under pressure a dam-bar part between a pair of planes which are fit in the wavelike form at regular intervals, thereby securing the flatness of a lead frame.

CONSTITUTION: A jig 22 consists of upper and lower dies 24 and 26 and the above dies have a pair of planes 24a and 26a which are fit in the wavelike form. After punching a lead frame 10, a dam-bar part 12 is put under pressure between a pair of the planes 24a and 26a which are fit in the wavelike form at regular intervals. Then, bending corrective force is applied to the part 12 at a short pitch in the cross direction of the lead frame 10. Accordingly, such an arrangement makes the fluctuations in respective surface stresses of both upper and lower planes which cause a trough warp phenomenon in the lead frame 10 small and makes it possible to secure the flatness of the whole lead frame 10. Occurrences of connection failure are thus prevented.