PURPOSE: To improve the performance of a semiconductor device by a method wherein the joint between the frame is made narrower than the width of a socket plug.

CONSTITUTION: The lead frame is formed out of a four-side frame 1 forming a square in the perimeter. Inside two opposed sides of the frame 1, leads 2 extending to the center whose end is rectangular are joined at the tip of the plug 2a which is the outer lead of the lead 2. The tip 2b of the plug 2a of the outer lead that of the joint between the frame 1 is narrower than the other part of the plug 2a. The CERDIP type semiconductor device manufactured by using such a lead frame is equipped with outer leads narrower in the plug tip 2b. Therefore, the mounting substrate can smoothly be plugged into a socket.