PURPOSE: To prevent turning-up at the joints and reinforce the strength thereof by overlapping the end portions of lead frames, executing point-welding to the edge portions along the longitudinal direction and thermally bonding a resin tape with pressure.

CONSTITUTION: The edge portions of lead frames 2, 4 are overlapped and point weldings 10, 12 are carried out thereto at both sides thereof in the longitudinal direction. The step joint portions 52, 54 generated at the joint are covered with the resin tapes 14, 16 which are thermally and pressurizingly bonded. Thereby, the step joint portions are formed into the smooth gradient portions and the end portion of joint is no longer turned up. The bonding area can be adjusted as required. As the resin tape, a adhesive tape obtained by coating the acrylic bonding agent to the polyimide tape is desirable.