PURPOSE: To form a kneaded material having a stable characteristic, by comparing a differential electric power estimated value after the elapse of a predetermined time with a preset reference power time series value and operating the correction quantity of an operation variable for a kneader in order to compensate the difference between both values.

CONSTITUTION: The differential electric power value of a DC motor 2 is hourly detected during the operation of a kneader 1 by a differential electric power detection part 9 and a differential power value after the elapse of a predetermined time is estimated on the basis of the detected differential power value by the operation part 12 of a control circuit 8. This estimated differential power value is compared with a preset reference power time series value in a comparing part 13 and, when both values coincide, the correction of the operation variable for the kneader 1, that is, the rotational control of the DC motor 2 by a motor control apparatus 3, the wt. pressure control by an air pressure control apparatus 6 and the temp. control by a recirculation water supply apparatus 7 are not performed and the motor 2 is operated as it is.