A system and method for determining and controlling the mixture ratio of an air-fuel mixture supplied to an internal combustion engine in which the air-fuel mixture ratio is estimated from a maximum internal pressure of an engine cylinder. The estimated air-fuel mixture ratio is then positively ascertained on the basis of a determination result derived by a weighting processing calculation of input signals indicating various engine and vehicle operating conditions affecting the change in the air-fuel mixture ratio using a weight vector with a reference signal. The air-fuel mixture is controlled on the basis of the ascertained air-fuel mixture ratio. Furthermore, each parameter of the weight vector is learned and corrected in a direction toward which a correct percentage of the determination of the rich-side or lean-side air-fuel mixture ratio by the weight calculation is increased.