An injection method for making a hole in an injection molded plastic article comprising the steps of filling a mold cavity with a molten plastic and cooling the mold to effect gate seal. A first hole-making punch and a second punch, which are slidably inserted in a bore provided in the mold coinciding with the hole to be made in the article in an axial direction in which a movable mold half is opened and closed, are vibrated with high frequency after gate seal is completed to soften the solidified plastic between the two punches. The punches are then displaced with respect to the bore when the plastic is softened to make the hole. A second injection method comprises the step of making the hole in the molded article by sliding the punches during both dwelling and cooling subsequent to the completion of dwelling after filling the mold with the molten plastic and before gate seal is effected. While said punches are subject to vibration with high frequency or slow reciprocating motion, the side surfaces of the punches are contacted against the hole inner surface to obtain a good cut surface of the hole in the molded article.