Disclosed are an ignition timing control device and ignition timing control system capable of easily performing ignition timing control on an internal combustion engine so as to suppress the occurrence of engine knocking even in the case where the internal combustion engine is not equipped with ignition timing control and knocking suppression functions. An ignition timing control device (31) has a knocking detection unit (41) and an ignition timing adjustment unit (43). 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 (31) outputs to an igniter (33) an adjusted ignition signal as adjusted (corrected) by an adjusted ignition timing determination process (step 630). The ignition timing control device (32) outputs to the igniter (33) a reference ignition signal (A) as it is outputted from an internal combustion engine control unit (37) without adjustment (correction) (step 640) in a condition that the operation state of the internal combustion engine is not suitable for adjustment of the ignition timing.