PURPOSE: To provide stabilized foaming of a highly foaming fluorocarbon resin coating layer by extruding fluorocarbon resin kneaded with fluorocarbon gas over an outside circumference of a conductor to execute coating thereof while at the same time housing it in a pressure vessel of high temperature and high pressure and lowering the temperature as well as pressure thereof in a required state.

CONSTITUTION: A fluorocarbon resin coating layer kneaded with fluorocarbon gas is extruded over an outside circumference of a conductor 4 by a crosshead 3 and while coating is being executed it is extruded into a pressure vessel 7 of high temperature and high pressure. An end thereof is gripped by a clip 8 and pulled with fixed force along a rail 10 of a moving table 9. Fluorocarbon gas is pressure filled into the chamber 7 and the inside thereof is heated and retained at about 320°C equal to temperature of the resin by means of a heater 11. A coated wire 1a is cut by a fixed length and simultaneously with opening of a valve 14 of pneumatic piping 13 a valve 16 of fluorocarbon gas piping 15 is closed and further a valve 19 of cooling water piping 18 is opened to spray cooling water. Control of the valves 14 and 19 can lower temperature and pressure along a required control curve.