An apparatus for controlling air-fuel ratio of an internal combustion engine having: a three-way catalytic converter; an oxygen sensor, which is installed at the upstream side of said catalytic converter in respect to exhaust gas flow direction, for generating a switching signal showing rich/lean relative to a certain air-fuel ratio; and an air-fuel ratio sensor, which is installed at the downstream side of said catalytic converter, for generating a linear output signal corresponding to the air-fuel ratio, wherein the apparatus includes a deviation calculation unit for calculating deviation between air-fuel ratio measured by the air-fuel ratio sensor, and target air-fuel ratio; and a feedback control unit of the air-fuel ratio for carrying out feedback control of air-fuel ratio based on the deviation calculated by a deviation calculation unit, and the output signal of the oxygen sensor.