PURPOSE: To provide a lead frame wherein the close contact strength of a semiconductor sealing resin is enhanced and a moisture-resistant property at the inside of the resin is enhanced.

CONSTITUTION: Grooves 4, 5 are formed at the resin-sealed part of a semiconductor mounting part 2 in a long-belt plate material. The grooves 4, 5 are formed to be narrow dovetail groove shapes whose open part is narrower than the width of the bottom part. The groove 4 is formed in such a way that a first groove 7 is formed by a rolling roller in a process to roll the long-belt plate material, that a second groove 8 is formed by compressing the first groove 7 from its upper part by a roller which is wider than it and that edges 4a of the first groove 7 are pushed out to the inside. A lead frame 1 is punched from the long-belt plate material having the groove 4, and a plurality of semiconductor mounting parts 2 and leads 3 connected to semiconductors by conductors are formed continuously in the lengthwise direction. The groove 5 is formed in such a way that a first groove 9 is formed when the lead frame 1 is punched and worked, that the first groove 9 is compressed from the upper part by a punch which is wider than it and that edges 5a of the first groove 9 are pushed out to the inside.