PURPOSE: To prevent trouble from arising due to deformation or crushing of a pipe-shaped inside sheath during manufacturing processes by installing a pressure-reducing molding device between an extrusion molding device and a cooling molding device so as to reduce the pressure of the inside sheath outwards, for swelling the sheath.

CONSTITUTION: Plastic material is extruded around the outer periphery of an inside conductor 1, and then a helical rib 22 and a pipe-shaped inside sheath 21 located around the outer periphery of the rib 22 are extruded and molded simultaneously, and then cooled and hardened to form an insulator. Then a pressure-reducing molding device 11 is installed between an extrusion molding device 6 and a cooling molding device 7 and the pressure of the pipe-shaped inside sheath 21 is reduced outwards by the device 11 so that the sheath is swelled. The pressure inside the inside sheath 21 may therefore be converted to atmospheric; pressure; in the case of taking up an insulator 2 onto a take-up drum 10, cutting of the insulator 2 will thus not cause such a trouble that the pressure inside the insulator 2 drops abruptly to cause the inside sheath 21 to be crushed between the extrusion molding device 6 and the cooling molding device 7.