PURPOSE: To reduce wear and tear of a cylinder or a screw by reducing starting torque of the screw without giving excessive compression force to raw materials of synthetic resin, by making a cylinder and a screw into a tapered state wherein the lower the inside diameter of the cylinder and the outside diameter of the screw are moved to downstream sides the larger they become and they are formed into 1.1W1.5 times of those at upstream sides of them respectively.

CONSTITUTION: Cross-sectional shapes of several lines of grooves 12 in the direction of an axis provided on places at some lengthy measurements (2W5 times of the inside diameter of a cylinder normally) from a supply port 11 in a downstream direction of the inside of the cylinder 10 can be either an angular or semi-circular groove. The inside diameter of the cylinder 10 is made uniform extending over the whole length of the section of a hopper ground A, a tapered part 13 is being formed in a manner of expanding gradually the inside diameter of the cylinder 10 in a downstream direction of a barrel 10 from a downstream end part of the hopper ground section A further, the tapered part 13 is being formed so as to realize relation in the form of an expression of D1=D0×(1.1W1.5) when the inside diameter at an upstream end of the tapered part 13 and the inside diameter at the downstream side of the tapered part 13 are given as D0 and D1 respectively, a length o the section B of the tapered part 13 is being kept about identical with that of the foregoing hopper ground section and the hopper ground section A and the section B of the tapred part B are overlapped to some extend each other.