PURPOSE: To decrease a scrap part of an insulating tape by cutting the insulating tape along their perforations to form a conductors exposed section while inter mittently retarding the travel speed of the insulating tape than that of conductors on their travels before the insulating tape is stuck to the conductor.

CONSTITUTION: Insulating tapes 2 each having a perforation 2a made therein by a perforating device 12 are respectively mated longitudinally with both surfaces of each of plural conductors 1 traveling forward in parallelly arranged relation together with a space held therebetween at a point of time when they pass between guide rolls 14. Braking by brake rolls 13 is applied to the travel of the insulating tapes 2 fed to the both surfaces of each of the conductors 1 on their travels when the tapes pass or have passed between preheating rolls 15 so as to retard the travel speed of each of them or to stop the travel, so that the sections of the insulating tapes 2 positioned before the parts of their perforations 2a are stuck respectively to the sections of the tape and the conductors 1 by means of main heating rolls 16 and the insulating tapes 2 are consequently cut respectively at the parts of their perforations 2a where the tensile strength is weak to move only the conductors 1 faster, whereupon a conductors-exposed section 1a is formed.