PURPOSE: To enhance the yield and quality of a lead frame by applying etching to an inner lead section to make a part or whole of the inner lead section thinner than the other sections in a lead frame where a metal plate is treated for patterning by photoetching to form the inner lead, outer lead, etc.

CONSTITUTION: Both sides of a metal plate 150μm thick are coated with photoresist 8 and temporarily dried. Next, the front surface of the plate is treated for exposure to form a particular pattern (a) containing an inner lead, outer lead, die pad, and dam bar, while the rear surface is treated for exposure to form a pattern that the pattern (a) is laid on another pattern that the inner side of waved line section 5 will slip off when development is carried out, followed by developing and drying the plate. Next, both sides of the plate are treated for etching by about 50μm each. In this process, a part on the rear side of the plate is the part where the neighborhood of the tip of the inner lead and the die pad will be prepared and will be made thinner than the other part.