To prevent an engine stall, by controlling ignition timing according to an extremely-low-speed rotational frequency at a more delayed angle than that at idling rotation, when the engine rotational frequency decreases to the extremely-low-speed rotational frequency lower than the idling frequency in the condition that a throttle valve a aperture is substantially the full opening.

In the case of switching an operation lever to the back running position during running ahead, when the engine rotational frequency decreases to the extremely-low-speed rotational frequency lower than the idling frequency by rotation of the operation lever to a neutral position, ignition timing is controlled to correspond to the engine rotational frequency and elapsed time detected at that time. In other words, as soon as the engine rotational frequency decreases to the extremely-low-speed rotational frequency, ignition timing is controlled to an ignition timing T2, and then gradually to the neighborhood above the dead point (TDC) with a delay angle. Furthermore, when the engine rotational frequency rises with the passing of time, the ignition timing is progressed near an ignition timing T1. Thus, necessary rotation power can be secured, and engine stall can be avoided.