PURPOSE: To realize optimum knocking state at all times for reducing engine damage due to knocking during engine operation at a high speed or under a heavy load, and control an engine in such knocking state as making sufficient torque available in an operation condition where relatively large knocking is allowed.

CONSTITUTION: Regarding output from a knocking sensor 6 between the predetermined crank angles, a peak value is held with a peak hold circuit section 7 for detecting a knocking intensity value, and knocking evaluation level is generated with a control circuit 8, so that the logarithmic conversion value of the peak value has the predetermined distribution form. Then, the state of knocking is evaluated via a comparison between the evaluation level and the knocking intensity value. In this case, when an engine speed and load are higher, an evaluation level adjusting factor is set at a smaller value with the control circuit 8.