PURPOSE: To contrive to improve the dimensional accuracy of a molding and to realize more efficient molding cycle by a method wherein the cooling of a core pin is made possible without impairing the releasability of the core pin from the molding.

CONSTITUTION: Four stripes of cooling water jackets 40... are formed on the outer peripheral surface of a core pin 30 so as to circulate cooling water in the respective cooling water jackets 40 through cooling water outlets 41 and cooling water inlets 42, all of which are formed on an ejector sleeve 31. By circulating cooling water in the respective cooling water jackets 40, the over- heated core pin 30 is cooled down so as to maintain proper pin temperature. The cooling water supplied to the respective cooling water jackets 40 is sealed with respective O-rings 50 and 55, resulting in leaking no cooling water to the surroundings of the jackets.