To restrict an insulation coat from entering into stranded wires, reduce adhesive property, and improve abrasion resistance by providing an air cooling area between a conductor softener and a coating extruder so that a conductor is below a specific temperature.

A conductor 2 passing through a dice 6 of a stranded wire machine and being a stranded wire is fed to a coating extruder 8 through a conductor softener 7. Next, while the conductor 2 passes through a cross head 8a of the extruder 8, a PVC (poly chloride vinyl) composition in a molten state is supplied around there, coated, and protruded, and an insulation coated body 3 is formed. A wire 1 thus coated is wound through a cooling layer 9. Here, the air cooling area is provided between the conductor softener 7 and the coating extruder 8 so that the conductor is below 70°C. When the conductor 2 is thus air-cooled and the PVC composition is extruded and coated, intrusion of the insulation coat 3 into stranded wires of the conductor 2 is restricted, and adhesive force between the conductor 2 and the insulation coated body 3 is lowered. Thus, even if a load is applied so as to cause friction with the insulation coated body 3, the load escapes, and abrasion resistance is improved.