a plastic molding device for plastic molding a semiconductor element includes upper mold and lower molds , each mold having a plurality of cavities , injection passages for injecting plastic into the cavities , and eject pins for ejecting plastic molded semiconductor elements from the cavities ; a plastic injecting mechanism for injecting plastic into the cavities ; and a mold driving mechanism for driving the upper or lower molds to join and separate them . the plastic molding device further includes a pin driver for projecting the eject pin after injection of the plastic into the cavities is completed while the upper and lower molds are together . therefore , shrinkage deformation on a surface of the plastic molded product is prevented , whereby a plastic molded semiconductor device having a smooth surface is produced .