An ignition timing control system for an internal combustion engine having a plurality of cylinders in which at each of different load conditions determined by the operating conditions of the engine the ignition timing for each cylinder is periodically retarded to cause a nonknocking ignition and the resulting background vibration levels detected by a knock sensor are stored in a memory. During the normal operation, the vibration level detected by the knock sensor with respect to each cylinder is compared with the corresponding background vibration level stored under the same load condition to detect the presence of knocking and thereby to determine the proper ignition timing.
