An injection molding apparatus has a mold clamping device that includes a traction platen disposed below a pressure-receiving platen, a movable platen disposed above the pressure-receiving platen and configured to be moved up and down by a mold opening and closing mechanism, and tie bars extending downward from the movable platen and penetrating the pressure-receiving platen and the traction platen. The mold clamping device is configured to clamp molds disposed between the pressure-receiving platen and the movable platen. Each tie bar has a tooth portion at its lower end that engages with a tooth portion of a half nut connected to an underside of the traction plate. An injection device arranged on the movable platen includes a heating cylinder having a screw for feeding resin material through a nozzle to an upper one of the molds, and a purging cover encloses a space between the lower end portion of the heating cylinder and the movable platen.