A kneader with a vent function, including a kneading extruder, which permits the continuous viscous flow of material to be kneaded even at a high speed rotation to prevent the generation of cuttings and exhibiting sufficient venting capability. The kneader includes a barrel having kneading chambers defined therein, a rotor rotatably inserted into the kneading chambers, and having a feed portion on an outer peripheral surface thereof. A vent port is in communication with the kneading chamber of the barrel at a position around the feed portion. The feed portion in a vent zone includes a screw tip having a material feed surface with an outwardly curved surface in section.