A method for detecting and controlling the air-fuel ratio of a multicylinder internal combustion engine through an output of a single air-fuel ratio sensor installed at a confluence point of the exhaust system of the engine. The detection response delay is assumed to be a first-order lag and a state variable model is established. Further, the air-fuel ratio at the confluence point is assumed to be a sum of the products of the past firing histories of the each cylinder of the engine and a second state variable model is established. An observer is then designed to observe the internal state of the second model and the air-fuel ratio at the individual cylinders are estimated from the output of the observer. The deadbeat control is carried out by calculating a ratio between the estimated air-fuel ratio and a target air-fuel ratio. The calculated ratio is multiplied to a correction value at a preceding control cycle earlier by a number corresponding to the number of the engine cylinders.