PURPOSE: To improve plasticizing capacity, by a method wherein the first thread is formed in a feed and compression zones and the second thread, which is different from the first thread and possesses a pitch larger than that of the first thread, is formed in the compression zone and a metering zone.

CONSTITUTION: Plasticized resin fed to a compression zone B from a feed zone A is compressed, flows into a space between higher threads of the first and second threads 3, 4 according to rotation of a screw 2, ramified with a lower thread and the two streams are mixed with each other through a gap between the lower thread and a heating cylinder 5. Therefore, the plasticized resin is agitated. With this construction, kneading of the plasticized resin in the compression zone B is performed favorably. Furthermore, a resin material is fed into a metering zone C of the thread 4 whose pitch is larger from a feed zone A of the thread 3 whose pitch is smaller and plasticized with heavy force.