An ignition timing correcting system generates a reference signal at a reference angular position of a rotating internal combustion engine and an ignition signal leading it by a constant phase. In response to a control signal from a movable tap on a potentiometer, the ignition signal is normally retarded in phase to form a retarded ignition signal. The phase comparison of the two signals produces an advance or a retardation signal formed of a pulse train having a pulse-width corresponding to a phase difference between the two ignition signals. During the idling of the engine, the correcting signal drives forwardly or reversely an electric reversible motor to move the tap on the potentiometer to adjust the control signal. During the knocking of the engine, a knocking sensed signal is added to the control signal from the potentiometer to further retard the ignition signal. During the idling of the engine, the knocking sensed signal is disabled to retard the ignition signal.