Internal mixer

An internal mixer comprising:

• a mixing chamber having a mixing room;
• a material input portion that is located above the mixing chamber and allows a material to be kneaded formed by a polymer material to pass through and feed the material to be kneaded into the mixing room;
• a pair of mixing rotors rotatably disposed in parallel to each other in the mixing room and configured to knead the material to be kneaded;
• a weight disposed in the material input portion so as to be capable of pressing the material to be kneaded in the mixing room from above;
• a hydraulic circuit having a hydraulic cylinder connected to the weight;
• and
• a controller that controls the hydraulic circuit to perform a pressing motion of supplying hydraulic oil to the hydraulic cylinder so that the material to be kneaded is pressed by the weight and a motion for suppressing rapid rise of limiting an operation of the hydraulic cylinder so as to suppress an upward moving speed of the weight when the pressing motion is canceled.