An air-fuel ratio control apparatus for an internal combustion engine of the present invention determines whether or not a lean request has occurred, based on a comparison between a value correlating with an output value of the downstream air-fuel ratio sensor disposed downstream of the catalyst (downstream air-fuel ratio sensor output correlating value) and a predetermined lean request determining value, and determines whether or not a rich request has occurred, based on a comparison between the downstream air-fuel ratio sensor output correlating value and a predetermined rich request determining value. Further, the air-fuel ratio control apparatus calculates a total amount of oxygen (released oxygen amount) released from the catalyst in the rich request occurring period, and calculates an integrated value (present stored oxygen amount) of oxygen stored in the catalyst after a start of the lean request occurring period which follows the rich request occurring period. The apparatus changes the rich request determining value in such a manner that a point in time at which it is determined that the rich request has occurred comes much earlier as the present stored oxygen amount comes closer to the released oxygen amount.