PURPOSE: To prevent the occurrence of oil fume and a faulty product due to said occurrence by enclosing the contact part of the end of the extruding port of the die extruding molten thermoplastic resin with the molten resin by the gas inert to molten resin.

CONSTITUTION: A die 1 is exclosed by a shutting wall 5, and the gas inert to molten thermoplastic resin is introduced into the space 10 between the die 1 and the shutting wall 5 from a gas-introducing port 6. The contact part of the tip of the die lip with the molten thermoplastic resin extruded from said tip, exists in said space 10, and is enclosed by the gas inert to the molten thermoplastic resin. As the inert gas, nitrogen gas is preferable for the reason of cost and safety. As the molten thermoplastic resin, polyethylene, polypropylene, ethylene-α-olefin copolymer, ethylene-vinylester copolymer, ethylene-α,β-unsaturated carboxylic acid copolymer and metallic neutralized material, etc., are used.