An ignition timing control apparatus for an internal combustion engine wherein an ignition timing .theta. of at least one cylinder of the engine is retarded (or advanced) by a little amount .DELTA..alpha. to intentionary check the generation sensitivity of a knock detection signal, that is, the detection sensitivity of knocking, then a knock judging level is modified thereby to judge as to whether knock is produced or not on the basis of the modified knock judging level, in order to correct and control the ignition timing .theta. of the engine. As a result, irrespective of the non-uniformity of properties of the knock detection device and an internal combustion engine due to the deterioration with age or manufacturing errors, correct judgement of knocking can always be insured.