The invention includes a shielding portion having a vent hole for discharging a gas composition G generated from a molten resin R to a barrel cylinder and having a shielding steam holding chamber C formed in connection with the barrel cylinder, the shielding steam holding chamber communicating with the vent hole. A steam supplying portion is provided for supplying a steam S to the shielding holding steam chamber C, and a discharging port portion is provided for discharging the steam S and the gas composition G from the inside of the shielding steam holding chamber C. Thereby, oxidization or carbonization of the molten resin can be prevented. In addition, adverse influence to the natural system can be removed and a decrease in the cost of the apparatus can be realized.