An engine is operated with air-fuel ratio F/B control interrupted, by regarding an alcohol concentration value ALC1 inputted from outside as a tentative concentration value, and by detecting the rotation-speed fluctuation amount of an engine, the appropriateness of the tentative concentration value is determined. In the case where it is determined that the tentative concentration value is appropriate, an estimated alcohol concentration value ALC2 stored in a storage means is replaced by the tentatively changed alcohol concentration value ALC1.