An air-fuel ratio control apparatus of an internal combustion engine according to the present invention is provided with oxygen storage amount estimating means, downstream exhaust air-fuel ratio detecting means, maximum oxygen storage amount estimating means, and air-fuel ratio target setting means. The oxygen storage amount estimating means estimates an oxygen storage amount of an exhaust purifying catalyst, based on a history of an oxygen adsorption/desorption amount of the exhaust purifying catalyst located on an exhaust path. The downstream exhaust air-fuel ratio detecting means is located downstream of the exhaust purifying catalyst and detects an exhaust air-fuel ratio downstream of the exhaust purifying catalyst. The maximum oxygen storage amount estimating means estimates a maximum oxygen storage amount, based on an oxygen storage amount estimate when the exhaust air-fuel ratio detected is a predetermined air-fuel ratio. This permits effective utilization of oxygen occlusion capability of the exhaust purifying catalyst and improvement in exhaust purification performance.