There is provided a spark ignition engine having a low RON fuel tank to store a low RON fuel and a high RON fuel tank to store a high RON fuel, which can detect storage failure. It is judged that the two kinds of fuels are properly stored when differences between the base ignition timing and the practical ignition timing at different loads are identical. When the differences are not identical, for example, it is judged that the high RON fuel is stored both in the two fuel tanks when a difference between the proper mix fuel knocking-occur ignition timing and the practical ignition timing is decreased as the load is increased. When a difference between the proper mix fuel knocking-occur ignition timing and the practical ignition timing is increased as the load is increased, it is judged that the two kind of fuels are reversely stored when a difference of differences between the proper mix fuel knocking-occur ignition timing and practical ignition timing at predetermined two load is greater than a predetermined threshold, and it is judges that the low RON fuel is stored both in the two tanks when the difference of differences between the proper mix fuel knocking-occur ignition timing and practical ignition timing at predetermined two load is smaller than a predetermined threshold.