Method and apparatus is provided for calculating an optimum ignition timing for an internal combustion engine on the basis of values set in dependence on operating conditions of the engine. With a view to prevent ignition times in two successive cycles from differing remarkably from each other, the ignition time or angle for the current cycle is controlled with reference to the ignition time or angle in the just preceding cycle. As one example of the control, in the case where the ignition time electronically anticipated for the current cycle appears to have deviated from the preceding ignition time by more than a predetermined value, the anticipated ignition time is modified to result in the optimum ignition time by adding to or subtracting from the anticipated ignition time a predetermined value. Alternatively, the optimum ignition time may be given by a mean value of the anticipated ignition time and a plurality of the preceding ignition times.