PURPOSE: To obtain a screw that is easy to grasp unmelted pellets and excellent in kneading and melting properties by providing a mixing section in which the section is polygon and composed of a polyhedron having a flight in which forward and backward leads are continued at the end of the screw.

CONSTITUTION: A mixing section (b) and a backward flow prevention device (c) are provided at the end of the metering section (a) of the end section of the screw 1. The mixing section (b) is composed of polyhedrons 2, 2a having polygonal sections. The flights 3, 3a dividing the bow-shaped space 15, 15a formed between the inner wall surface 13 of the cylinder and the polyhedrons 2, 2a lead forward on the side of the metering section (a) and lead backward on the side of the backward flow prevention mechanism (c) and fits the key 19 to the coupling 8 so that the polyhedrons 2, 2a keep the normal position. The backward flow preventing section (c) is composed of a sheet 4, a check ring 5, a retainer sheet 6 and a ring 7.