PURPOSE: To uniformize the viscosity of the molten polymer near the spinneret, and to improve the filtering efficiency, by placing a porous plate which generates Joule heat by electrical current nearly parallelly to the surface of the spinneret opposite to the extrusion side interposing a gap therebetween.

CONSTITUTION: The molten polymer supplied to the upper block 2 is passed through the porous plate 4 to obtain molten liquid having low impurity content and having uniform temperature by the filtering and heating actions of the plate, and the liquid is extruded through the nozzle 6' of the spinneret 6. The porous plate 4 is electrified through the terminals 5 using a slider for voltage control and a transformer for voltage transformation, and the required amount of Joule heat is generated. The porous plate is e.g. a laminate manufactured by sintering three sheets of stainless mesh.