PURPOSE: To improve dimensional accuracy by forcibly cooling the outside of a molding resin with a refrigerant after extrusion molding while also cooling the molding resin from the inside by a cooling section in a mold extending section.

CONSTITUTION: An internal mold 3 is extended from the position of combination with an external mold 2, changed into a mold extending section 3A and projected into a refrigerant 5. A cavity is formed into the mold extending section 3A as a cooling section 4, a refrigerant (such as water) is introduced from a capillary 4a communicated with the cavity, and discharged to an upper section by another capillary from the bottom of the cavity, and the refrigerant is circulated. A molten resin 1 is passed through a mold clearance and extruded into the refrigerant 5 under the state in which the external mold 2 and the internal mold 3 are combined and the mold extending section 3A is projected into the refrigerant 5. An extruded molding resin 10 is colored forcibly from the inside and the outside when it is passed through the mold extending section 3A, and cooled completely.