motor 62 may be damaged. To prevent such overload, the electric current flowing through the motor 62 is detected by the meter relay 72, and when the detected current value exceeds the rated value of the motor 62, the exciting current for the upper solenoid valve 65 is cut off by the meter relay 72. As a result, the communication of the feed port 63 to the upper port 52 is cut off and the upper port 52 is opened to the atmosphere, whereby the compressed air present in the upper portion of the interior of the cylinder 51 is discharged into the atmosphere. Consequently, the closing force for the upper opening of the case 59 now corresponds to only the weight of the weight 56, so thata large lump of the material to be kneaded is pushed up with the operation of the kneader blades 61 and the internal volume of the case 59 increases, thus facilitating the movement of the material being kneaded, and hence the load imposed on the motor 62 is reduced. When the electric current flowing through the motor 62 has become smaller than the rated current value of the motor, the interception of the exciting current for the upper solenoid valve 65 is discontinued to reestablish communication of the upper port 52 with the feed port 63, whereby the passage leading to the atmosphere is cut off. Consequently, compressed air flows into the upper portion of the interior of the cylinder 51, so that the weight 56 strongly closes