To carry out optimal control per fuel in a low cost by carry out learning control, and judging fuel in the case where a range in which the learning control is carried out exceeds a prescribed rate in all ranges in which feed back control is carried out, in a device for feed back controlling an air-fuel ratio to a target air-fuel ratio.

In a control means 132, when a program for fuel judgment is started, primary operation is carried out after a data is cleared. The operation is carried out in a prescribed operating condition in a prescribed range, and a feed back data is set to 0. It is judged whether it is an operation by a feed back range or not, in the case of 'YES', feed back control and learning control are carried out. It is judged whether a range in which the learning control is carried out exceeds a prescribed rate in all ranges in which feed back control is carried out or not, in the case of 'YES', the judging value of fuel judgment is retrieved. In the case where the judging value is -3 and less, special gasoline is judged, and in the case where the judging value is 3 or more, regular gasoline is judged.