An air-fuel ratio control device for an internal combustion engine having an intake passage and an exhaust passage with a three way catalytic converter. The device comprises a fuel injector for injecting fuel and an air-fuel ratio detector for detecting an air-fuel ratio in the engine cylinder, which is arranged in the exhaust passage upstream of the three way catalytic converter. A first estimation device estimates an amount of intake air actually fed into the engine cylinder. A first decision device decides a target amount of fuel which must be fed into the engine cylinder to realize the stoichiometric air-fuel ratio, on the basis of the amount of intake air estimated by the first estimation device. A second estimation device estimates an amount of fuel actually fed into the engine cylinder, on the basis of the air-fuel ratio detected by the air-fuel ratio detector, and the amount of intake air estimated by the first estimation device. A second decision device decides an amount of fuel actually injected by the fuel injector such that a difference between the amount of fuel estimated by the second estimation device and the target amount of fuel decided by the first decision device, and a time integration value of the difference are made to converge to "0" at the same time.