PURPOSE: To minimize the extension of a kneading cycle time and to reduce the consumption of air.

CONSTITUTION: In an internal kneader 1, a floating weight 8 is set vertically movably by an air cylinder 7 in a hopper 6 installed next to a kneader main body 2. An air blower means 14 for cleaning the weight 8 is fixed to the back door of the hopper 6 at a position close to the lower limit position of the weight 8 or above the lower limit position and at or below the midpoint.