To provide a kneading screw which can reduce such a phenomenon that a disproportional load is imposed onto a shaft of the kneading screw without degrading kneading performance of the kneading screw, a twin screw extruder with the kneading screw, and a method for assembling the kneading screw. In the kneading screw, each of the kneading rotor segments has at least one of a spiral kneading blade or a kneading blade extending in parallel with the axial direction, and is incorporated in the kneading screw such that top blade portions of the kneading blades at the adjacent ends of the kneading rotor segments adjacent to each other in the axial direction of the kneading screw are shifted in phase to each other, and such that the top blade portions of the kneading blades at the respective central positions in the axial direction of the adjacent kneading rotor segments are shifted larger in phase than the case where the top blade portions of the adjacent ends of the adjacent kneading rotor segments are equal in phase to each other.