PURPOSE: To enable a knocking preventing means to be not actuated in the case of an operator-desired speed variation by arranging a device for identifying knocking oscillations and counter acting knocking oscillations by corresponding torque variations.

CONSTITUTION: Speed outputted from an engine 11 is detected by a speed sensor 12 via a sensor wheel 10 and supplied to a microprocessor 13 for speed evaluation and speed variation detection. Torque is reduced when speed is increasing and increased when torque is decreasing. Particularly, the device 13 identifies knocking oscillation by detecting the difference between variations of sequential succeeding detected speed values. Knocking oscillation is counter acted by the corresponding torque variation. Thus, the speed variation due to knocking and the speed variation desired by the operator is discriminated. In the later case, a knocking preventing means is not actuated.