PURPOSE: To shorten the time required to change resin and save at the same time the amount of resin consumption by attaching detachably a plugging lid to the vent hole of a vent type injection molding machine.

CONSTITUTION: In a vent type injection molding machine consisting of a screw 2 comprising a first feed zone, first compression zone, first metering zone, vent zone, second compression zone, and second metering zone, a heating cylinder 1 that has vent holes 3 at the position corresponding to said vent zone, and a plugging lid 10 that is mounted detachably in said vent hole 3 are closed by the plugging lid 10, the time to change resin and amount of resin consumed for the change can be remarkably saved.