An air-fuel ratio control apparatus of an internal combustion engine comprising: a first exhaust gas purifying catalyst disposed in an exhaust passage; a second exhaust gas purifying catalyst disposed downstream of the first exhaust gas purifying catalyst; first air-fuel ratio acquiring means provided upstream of the first exhaust gas purifying catalyst, for acquiring an air-fuel ratio of exhaust gas; second air-fuel ratio acquiring means for acquiring an air-fuel ratio of the exhaust gas flowing into the second exhaust gas purifying catalyst; and air-fuel ratio controlling means controlling an air-fuel ratio of the exhaust gas of the internal combustion engine such that it becomes a target air-fuel ratio, said air-fuel ratio being acquired by the first air-fuel ratio acquiring means, and controlling the air-fuel ratio acquired by the second air-fuel ratio acquiring means, wherein the air-fuel ratio controlling means comprises: lean control means controlling the air-fuel ratio of the exhaust gas of the internal combustion engine until the second exhaust gas purifying catalyst becomes lean after completion of a fuel quantity increasing operation of the internal combustion engine, wherein the catalyst is lean when occluding oxygen in the catalyst until it reaches the maximum oxygen occlusion quantity; and intermediate lean control means performing, at least one time, control to change the air-fuel ratio of the exhaust gas of the internal combustion engine to a lean air-fuel ratio within the range enough to make the first exhaust gas purifying catalyst lean and not enough to make the second exhaust gas purifying catalyst lean between the fuel quantity increasing operation and the air-fuel ratio control by the lean control means, and performs an air-fuel ratio control by the lean control means during an idle operation of the internal combustion engine, wherein the air-fuel ratio controlling means performs an air-fuel ratio control by the intermediate lean control means during a substantially steady operation in a partial load region of the internal combustion engine. a first exhaust gas purifying catalyst disposed in an exhaust passage; a second exhaust gas purifying catalyst disposed downstream of the first exhaust gas purifying catalyst; first air-fuel ratio acquiring means provided upstream of the first exhaust gas purifying catalyst, for acquiring an air-fuel ratio of exhaust gas; second air-fuel ratio acquiring means for acquiring an air-fuel ratio of the exhaust gas flowing into the second exhaust gas purifying catalyst; and air-fuel ratio controlling means controlling an air-fuel ratio of the exhaust gas of the internal combustion engine such that it becomes a target air-fuel ratio, said air-fuel ratio being acquired by the first air-fuel ratio acquiring means, and controlling the air-fuel ratio acquired by the second air-fuel ratio acquiring means, wherein the air-fuel ratio controlling means comprises: lean control means controlling the air-fuel ratio of the exhaust gas of the internal combustion engine until the second exhaust gas purifying catalyst becomes lean after completion of a fuel quantity increasing operation of the internal combustion engine, wherein the catalyst is lean when occluding oxygen in the catalyst until it reaches the maximum oxygen occlusion quantity; and intermediate lean control means performing, at least one time, control to change the air-fuel ratio of the exhaust gas of the internal combustion engine to a lean air-fuel ratio within the range enough to make the first exhaust gas purifying catalyst lean and not enough to make the second exhaust gas purifying catalyst lean between the fuel quantity increasing operation and the air-fuel ratio control by the lean control means, and performs an air-fuel ratio control by the lean control means during an idle operation of the internal combustion engine, lean control means controlling the air-fuel ratio of the exhaust gas of the internal combustion engine until the second exhaust gas purifying catalyst becomes lean after completion of a fuel quantity increasing operation of the internal combustion engine, wherein the catalyst is lean when occluding oxygen in the catalyst until it reaches the maximum oxygen occlusion quantity; and intermediate lean control means performing, at least one time, control to change the air-fuel ratio of the exhaust gas of the internal combustion engine to a lean air-fuel ratio within the range enough to make the first exhaust gas purifying catalyst lean and not enough to make the second exhaust gas purifying catalyst lean between the fuel quantity increasing operation and the air-fuel ratio control by the lean control means, and performs an air-fuel ratio control by the lean control means during an idle operation of the internal combustion engine, wherein the air-fuel ratio controlling means performs an air-fuel ratio control by the intermediate lean control means during a substantially steady operation in a partial load region of the internal combustion engine.