To sharply enhance the dispersibility and distribution properties in kneading.

The screw shaft 5 arranged in a cylinder 4 comprises screw blades 8 and 9 extending toward the front end side of the cylinder 4 on the outer periphery of a base shaft 7 in two rows in a double spiral state. A plurality of interrupted parts 11 are formed to one screw blade 8 by interrupting the spiral of one screw blade 8 and a plurality of rows of sub-screw blade pieces 13, which extend in parallel to the other screw blade 9, are provided to the interrupted parts 11.