PURPOSE: To form an insulation coating film of a thin film type having ≤50μm thickness by previously bringing an oxidizing agent into contact with the surface of a conductor consisting of copper or a copper alloy to form a copper oxide film, and then forming the insulation coating film on this film.

CONSTITUTION: An oxidizing agent is previously brought into contact with the surface of a conductor consisting of copper or a copper alloy to form a film of copper oxide and thereafter, the insulation coating film is formed on the copper oxide film. The copper oxide film is more adequately the fine granular copper oxide film essentially consisting of cupric oxide and contg. cuprovs oxide. The conductor surface is adequately subjected to the pretreatment for removing the copper oxide film already formed on the conductor surface by the effect of the oxygen, etc., in the atmosphere before the copper oxide film is formed on the conductor surface. The thickness of the insulation coating film mentioned above is preferably ≤50μm and the breakdown voltage thereof is preferably ≥5000V.