PURPOSE: To provide a heat resistant electric wire endurable against use under, a high temperature environment by forming a silica film in a conductor by a specific method, and thereafter performing extrusion of an insulating layer, so as to prevent the conductor from its diffusion and corrosion.

CONSTITUTION: A heated conductor 1 is immersed in an isocyanate silane liquid, to produce a silica film 2 on the conductor 1, and the periphery of this silica film 2 is extrusion-coated with a heat resistant insulating resin 3. The conductor 1, after it is heated to a 100 to 300°C temperature, is immersed in the isocyanate silane liquid. As isocyanate silane, for instance, tetraisocyanate silane (186 to 188°C boiling point), phenylsirilisocyanate silane (252 to 254°C boiling point), etc., are listed.