PURPOSE: To obtain a resin form uniform and not broken easily by a method wherein a core pin is inserted within a cavity beforehand and the resin is filled into the cavity under injecting the resin from the tip end of the core pin and withdrawing the core pin from the cavity.

CONSTITUTION: The base of the core pin 6 is secured to a fixed side retainner plate 5 connected to the nozzle of an injection molding machine and the core pin 6 is protruded. A penetrating hole 7 is provided in this core pin 6 and the hole 7 is communicated with a resin pouring port 1a at the side of the nozzle. The cavity 8a is provided in a cavity plate 8 slidable with respect to a guide pin 2. At first, the core pin 6 is being fitted within the cavity 8a, then, the cavity plate 8 is moved into the direction opposite to the nozzle under injecting the resin into the cavity 8a from the penetrating hole 7 and the resin is filled in the cavity 8a to mold it. When the cavity plate 8a is moved further after the injected resin is cured, the form adhered to the tip end of the core pin 6 may be withdrawn from the cavity 8a.