An ignition timing control system for an internal combustion engine in which an angular position of the internal combustion engine is detected, and on the basis of the signal thus obtained by detection, the ignition timing of the internal combustion engine is determined by calculation in a computing device. A knocking condition is judged by a detection signal produced from a knocking detector of the internal combustion engine, and the ignition timing is corrected by the judgement of knocking. In view of the fact that the knocking occurs only during a certain period of the driving cycle of the internal combustion engine, a knocking condition is judged by comparing a detection signal produced from the knocking detector during the time in the absence of knocking with a detection signal produced during the time in the presence of knocking.