PURPOSE: To ultrasonically weld a flat conductor to another conductor while an insulating film layer is provided on an unwelded face.

CONSTITUTION: Flat conductors 11, 21 are pinched by the horn P of a ultrasonic welding machine W and an anvil A while the flat conductor 11 exposed on the face opposite to the face provided with an insulating film layer 12 is kept in contact with the flat conductor 21 exposed on the face opposite to the face formed with an insulating film layer 22. Projections P1, A1 provided on pressure faces of the horn P and the anvil A bite into the insulating film layers 12, 22 into contact with the flat conductors 11, 21, the horn P is vibrated, and the flat conductor 11 and the flat conductor 21 are welded. The troublesome work to provide the insulating films 12, 22 on the flat conductors 11, 21 is not required.