Internal mixer

An internal mixer comprising:

• a mixing chamber having a mixing room having an upper opening;
• a material input portion located above the mixing chamber and having an inner space communicating with the mixing room;
• an openable and closable hopper provided on one lateral wall of the material input portion and configured to be able to introduce a material of a material to be kneaded into the inner space of the material input portion;
• an exhaust passage connected to another lateral wall so as to communicate with the inner space through an exhaust port formed on the another lateral wall of the material input portion;
• a weight provided in the inner space;
• a driving device configured to generate a driving force for raising and lowering the weight and to be able to stop the weight at a lower position and at an exhaust adjustment position, the lower position being a position where the weight enters an upper opening of the mixing room while the weight is able to press the material to be kneaded in the mixing room from above, and the exhaust adjustment position being located above the lower position and closing at least the exhaust port by the weight;
• and
• a pair of mixing rotor configured to knead the material to be kneaded in the mixing room,
• wherein the weight has a shape that forms a gap between the weight and a peripheral edge defining the upper opening of the mixing room so that gas generated from the material to be kneaded can be discharged from the mixing room to the inner space when the weight is at the lower position.