In a method of controlling the point of ignition and/or air-fuel mixture of an i. c. engine, a signal circuit is used for emitting a signal indicating abnormal combustion or disturbance, and the first derivative of the spark current is used for the detection. According to the invention, the average peak value of the first derivative of the spark current is a reference value of the detection, and the actual signal of the derivative of the spark current is compared via a voltage divider and a comparator circuit with said average peak value, whereby detection is obtained when the signal from the voltage divider exceeds said average peak value.