An ECU is used in a motorcycle including an engine E, which combusts in a combustion chamber a blended fuel in which gasoline and ethanol are blended, and a fuel injection device for injecting a fuel with a predetermined fuel injection amount such that an air excess ratio in the combustion chamber is a first predetermined value when an ethanol concentration of the blended fuel is zero. The ECU executes O2 feedback control based on an O2 concentration derived from an output of an O2 sensor. Further, the ECU calculates an actual fuel compensation ratio by dividing a fuel injection amount after the O2 feedback control by a predetermined fuel injection amount, and estimates the concentration of the alternative fuel based on this actual fuel compensation ratio and a first predetermined value stored in the ECU.