Abstract: A method is disclosed in which an insulating film of an oxide of metal, of which a stranded conductor (30) is made, is formed on the surface of all strands in a stranded conductor, while the stranded conductor is held in the stranded form. In a step of oxidation-treating the strands of an electric cable the pressure of an oxidation treatment solution at the outer portion of the stranded conductor immersed therein is made higher than the pressure in the inner portion of the stranded conductor (30), causing the oxidation treatment solution (48) to be adequately penetrated from the outer portion into the inner portion of the stranded conductor to permit a desired oxide film to be formed on the surface of all the strands in the stranded conductor.