An ignition timing control device has a knocking detection unit and an ignition timing adjustment unit. In a condition that the operation state of the internal combustion engine is suitable for adjustment of the ignition timing, the ignition timing control device outputs to an igniter an adjusted ignition signal as adjusted (corrected) by an adjusted ignition timing determination process. The ignition timing control device outputs to the igniter a reference ignition signal as it is outputted from an internal combustion engine control unit without adjustment (correction) in a condition that the operation state of the internal combustion engine is not suitable for adjustment of the ignition timing.