Thermoplastic injection molding is applied in a wide variety of products. It is known that volatile components generated when resin melts cause structural defects during the thermoplastic injection process. Not only that, the pre-injection preparation work including drying the material and maintaining the metal mold, have become standard procedures to prevent these defects, but these procedures deteriorate productivity. Vent-type injection molding involves the use of cylinders featuring a vent hole at the center of the plasticizing cylinder. Although it is a conventional molding method, there are many issues including resin leakage from a vent hole and difficulty of material replacements. These issues prevented it from widespread application. Moreover, the vent-type plasticization process has not been examined theoretically or systematically. In order to maximize and generalize functions of vent-type molding, it is necessary to clarify the flow behavior of resin in the vent cylinder. In this study, we verified the flow behavior of resin in the vent-type plasticization cylinder through experimentation and simulation. In the simulation, using the flow analysis method, the filling rate inside the screw was determined by the pressure distribution inside the screw. In the experiments, the molding condition that causes venting up was verified by changing the screw rotation rates and the supply amount of the resin, for determining the filling rate of resin inside the screw. The filling rates obtained through the simulation and the experiment are almost the same. The result suggests that this simulation is very effective for predicting the filling rate.