PURPOSE: To prevent the stagnation and thermal decomposition of a resin at a part having no screw flights by extending the rear end parts of the screw fligthts on the leading end side of a screw to the boundary parts of a supply part, a compression part, a vent part and a weighing part.

CONSTITUTION: The leading end zone 1 (weighing part) of one of adjacent flight zones has four flights 11 and the root zone 2 (vent part) of the other one of the flight zones has two flights 21. The rear end parts of the screw flights 11 of the leading end weighing part 1 are extended to the boundary part 3 of the weighing part 1 and the vent part 2 every other one to form extension flights 4. Since the extension flights 4 are provided to the boundary part 3, propelling force is applied to a material resin and the transport speed of the material resin is increased and the stagnation thereof is eliminated.