An air/fuel ratio feedback control system with the adaptive controller. The adaptive controller has a system parameter estimator which identifies system parameters such that the error between the desired value and the controlled variable outputted from the plant decreases to zero. In the system, a basic fuel injection amount is determined at a feedforward system. The adaptive controller is placed outside of the basic fuel injection amount determining system and receives the desired value (air/fuel ratio) and the controlled value (detected air/fuel ratio) using the system parameters etc. The calculated feedback coefficient is multiplied to the basic fuel injection amount to determine a final injection amount to be supplied to the plant (engine). Variables necessary for the system parameter calculation are limited their change of range so as to make the controller to be realized on a low performance computer with a small word length.