PURPOSE: To reduce efficiently shock and shaking due to acceleration, by calcu lating the correctional amount of an ignition timing so that the difference be tween the smoothing rotational speed and the instantaneous rotational speed of an engine may become smaller, and also correcting the correctional amount on the basis of an idling operation condition.

CONSTITUTION: The instantaneous rotational speed of an engine is calculated at an operation unit 54 on the basis of a detection signal from a TDC sensor. Also, the filter operation of the detection signal from the TDC sensor is made at an operation unit 55, and a smoothing rotational speed opposing to the instan taneous rotational speed is calculated. In addition, a rotational variation which is obtained by subtracting the instantaneous rotational speed from the smoothing rotational speed, is calculated by an operation unit 56. Meanwhile, at a calculat ing unit 59, the basic ignition timing θB is corrected to a correctional ignition timing θC on the basis of detection information or the like from an intake air temperature sensor 18, an atmospheric pressure sensor 19 and a water temper ature sensor 43 or the like. In a calculating unit 60, a target ignition timing is calculated from the rotational variation band and the correctional ignition timing.