An ignition timing control system for an internal combustion engine, which is capable of ensuring both stability of control in a steady operating condition of the engine, and an excellent follow-up property of a controlled variable to a target value in a transient operating condition of the engine, even when the controlled variable contains a lot of high-frequency noise components. In the ignition timing control system, a maximum pressure angle-calculating section calculates a maximum pressure angle based on an in-cylinder pressure and a crank angle position. A target angle-calculating section calculates a target angle. A maximum pressure angle controller calculates a maximum pressure angle correction term with a control algorithm to which is applied a sliding mode control algorithm, using a value obtained by performing ε-filtering on a switching function, such that the maximum pressure angle converges to the target angle. The ignition timing is calculated by adding corrected ignition timing to the value.