A method for injection molding polyethylene terephthalate by employing a vent type injection unit as means for omitting preliminary drying the resin, wherein the unit does not develop a poor screw biting, and even when the molding material is an undried uncrystallized or crystallized PET, can feed a certain amount of the material at all times to injection mold a desired transparent molded form such as a preform, and use the preform to produce a thin-wall container at a low cost.