Apparatus and method for controlling the air fuel ratio of an internal combustion engine

The invention relates to an apparatus for controlling air-fuel ratio of an internal combustion engine having: a three-way catalytic converter (212) ; an oxygen sensor (213), which is installed at the upstream side of said catalytic converter (212) 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 (214), which is installed at the downstream side of said catalytic converter (212), for generating a linear output signal corresponding to the air-fuel ratio, wherein the apparatus include a deviation calculation unit for calculating deviation between air-fuel ratio measured by the air-fuel ratio sensor (214), 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 (213).