An ignition timing control system for an internal combustion engine that estimates the octane number of the fuel being used based on the state of engine combustion knocking and controls the ignition timing to a value not exceeding the knocking limit. The basic ignition timing is set along the knocking limit of fuel having least octane number and the maximum advance amount is set between the knocking limit and the second knocking limit of fuel having maximum octane number. Octane number is estimated from the occurrence of knocking. On the other hand, upper limit of the estimated octane number is set based on the MBT of the engine such that the estimated octane number is limited with the upper limit. Advance rate is determined based on the estimated octane number and is multiplied to the maximum advance amount to determine an ignition timing correction amount. The ignition timing is finally determined from the basic ignition timing and the ignition timing correction amount. A learning control is conducted for the octane number estimated and the estimated octane number is limited within the learned value.