PURPOSE: To provide the screw segments of the screw of a biaxial extruder, which makes the change of the number of flights possible so as to realize the necessary feed of the predetermined material or the like without developing stagnation in the flow of the material.

CONSTITUTION: In the screw segments 13 of a biaxial extruder, which are rotated in the barrel of the extruder so as to melt and knead material while moving in the barrel and fitted onto the screw shafts 23, which engage completely with each other, and has a flight 21, a transition part D, in which the number of flights at the entrance side of the screw segment 13 is different from the number of flights at its exit side and the height of a flight 14, the number of which is different from each other at the entrance side and at the exit side in the screw segment 13, is formed.