An ignition timing control system for an internal combustion engine, which is capable of reducing the capacity of a memory that stores data used in controlling ignition timing, thereby reducing manufacturing costs. An ignition timing control system that controls ignition timing of an internal combustion engine calculates a maximum torque parameter indicative of a maximum torque that the engine can output when the engine is at the detected rotational speed, according to the detected rotational speed, calculates an output torque parameter indicative of an output torque being output from the engine, calculates a torque ratio as a ratio between the output torque parameter and the maximum torque parameter, and determines the ignition timing according to the engine speed and the torque ratio.