A kneading adjustment mechanism 80 includes a pair of movable members 81 and 82 each having a flow path forming surface 85 constituting a part of a flow path 20 through which kneaded molten resin is conveyed and a screw 84 arranged between the flow path forming surfaces 85 of the respective movable members 81 and 82. At least one of the pair of movable members 81 and 82 can operate such that a gap between the flow path forming surface 85 and the screw 84 increases or decreases, and at least one or a plurality of protrusions 86 is provided on an outer periphery of the screw 84.

A gearbox assembly for a twin-screw extruder comprises a speed change gearbox for adjusting a screw shaft speed of two screw shafts of the twin-screw extruder, a distribution gearbox allowing the screw shafts to be coupled thereto, and a speed reduction gearbox. The speed change gearbox and the speed reduction gearbox are configured such as to be separable from each other, wherein the speed change gearbox is arranged at a driving end while the speed reduction gearbox couples the speed change gearbox to the distribution gearbox. Due to the fact that the speed change gearbox is configured as an individual gearbox unit arranged at the driving end, an adjustment of the screw shaft speed is easily possible by replacing the speed change gearbox. As a result, a high flexibility and productivity are obtained in the operation of the twin-screw extruder.