The rate of variation in the rotational period of an engine is determined by the use of a pulse signal generated according to the rotation of an engine crankshaft, and the value of the rate of variation thus determined is compared with a previously set value of judgement of the rate of variation. When the rate of variation is under the judged value, ignition is performed at an advance or retard timing given by calculation; and when the rate of variation exceeds the judged value, an ignition signal is used as a reference crank position signal in place of an ignition command signal obtained by calculation.