A plastic extrusion method comprising: preparing a cylinder comprising a transport section cylinder, a kneading section cylinder and a releasing section cylinder, wherein a screw is installed in the cylinder and comprises a transport section screw, a kneading section screw and a releasing section screw; supplying a plastic raw material into the cylinder from a hopper of a raw material supplying portion at an upstream side of a transport section cylinder; plasticizing and kneading the plastic raw material; and extruding the plasticized and kneaded material from the releasing section cylinder, wherein at the upstream side of the cylinder, an inner hole of the cylinder has an upper polygonal groove formed over the entire length of the inner hole beginning from the downstream end of the hopper, wherein at the downstream side, a transition cylinder is arranged between the transport section cylinder and the kneading section cylinder, wherein the transition cylinder includes a lower part and an upper part of the inner hole, wherein the upper part is configured as the upper polygonal groove configured to form a down-slope towards the downstream side so as to disappear eventually, and wherein in the step of supplying the plastic raw material, the plastic extrusion method comprises the step of releasing at least inert gas components and volatile components toward the hopper when the plastic raw material passes through the upper polygonal groove, characterized in that the lower part of the inner hole of the transition cylinder has a semi-circular shape and the upper part comprises at least four grooves. preparing a cylinder comprising a transport section cylinder, a kneading section cylinder and a releasing section cylinder, wherein a screw is installed in the cylinder and comprises a transport section screw, a kneading section screw and a releasing section screw; supplying a plastic raw material into the cylinder from a hopper of a raw material supplying portion at an upstream side of a transport section cylinder; plasticizing and kneading the plastic raw material; and extruding the plasticized and kneaded material from the releasing section cylinder, wherein at the upstream side of the cylinder, an inner hole of the cylinder has an upper polygonal groove formed over the entire length of the inner hole beginning from the downstream end of the hopper, wherein at the downstream side, a transition cylinder is arranged between the transport section cylinder and the kneading section cylinder, wherein the transition cylinder includes a lower part and an upper part of the inner hole, wherein the upper part is configured as the upper polygonal groove configured to form a down-slope towards the downstream side so as to disappear eventually, and wherein in the step of supplying the plastic raw material, the plastic extrusion method comprises the step of releasing at least inert gas components and volatile components toward the hopper when the plastic raw material passes through the upper polygonal groove, characterized in that the lower part of the inner hole of the transition cylinder has a semi-circular shape and the upper part comprises at least four grooves.