PURPOSE: To obtain a high insulating characteristic, by winding presser bar tape or sheet over the self-welding tape wound layer to the cable external semiconductive layer so as at least to cover a steppedly peeled cable insulator with it for heat treatment, and then by removing the presser bar tape or sheet.

CONSTITUTION: From the wound layer of self-welding insulating tape 5 to each cable external semiconductive layer 7 so as at least to cover on each steppedly peeled cable insulator 6 with it, presser bar tape or sheet 8 is wound around for heat treatment. As it is heated, the self-welding insulating tape 5 or cable insulator 6 will expand. The expanded portion will fill up the gap appearing on the interface between the wound layer of the self-welding insulating tape 5 and the cable insulator 6 and a riser section. After that, it is cooled, then the presser bar tape or sheet 8 is removed, so that reinforced insulating layer without gap excellent in the insulating characteristic will be formed.