An air-fuel ratio controller which detects oxygen content of the engine exhaust gases, compares the detected value with an objective oxygen content corresponding to an objective air-fuel ratio and integrates the deviation therebetween, smoothes the integrated values to absorb errors in control caused by variations in an injector or the like, and controls the air-fuel ratio in accordance with such integrated and smoothed values. The air-fuel ratio controller always controls the air-fuel ratio properly by stopping the smoothing of the integrated values when, for example, the detected oxygen content is disordered, and the detected values do not vary remarkably for more than a predetermined period.