This specification discloses an ignition timing control system for an engine having a countermeasure for an abnormal condition of a signal. The ignition timing control system according to this invention comprises: a first signal generator (3, 13) for generating a first signal (3a) at intervals of a predetermined crank angle during an engine is rotated n times ( n is an arbitrary positive number), this first signal (3a) including a marked signal (Mark-A) generating to a particular cylinder each time the engine is rotated n times; a second signal generator (1, 13) for generating a second signal (1a) of the number equal to that of all cylinders of the engine at intervals of substantially an equal crank angle in synchronism with the first signal (3a), this second signal (1a) including a signal of a form which is distinguishable from other second signals (1a) and which occurs in synchronism with the marked signal (Mark-A); a unit (14) for producing an ignition signal (IGN) in response to the first signal (3a); a distributor (24) for receiving the ignition signal and distributing the ignition signal (IGN) in response to the marked signal (Mark-A) of the first signal (3a) to each cylinder in a predetermined ignition order; a unit (CPU, Fig. 7) for detecting an abnormal condition of the first signal (3a); and a unit (CPU, Fig. 8, Fig. 9) for distributing the ignition signal to each cylinder in a predetermined ignition order in response to the signal synchronised with the marked signal of the second signal in place of the marked signal when the abnormal condition of the first signal (3a) is detected.