An apparatus for controlling an air-fuel ratio in a lean burn internal combustion engine, such that three different air-fuel ratio zones are set, the three zones being an ultra lean zone with a low load, a medium lean zone with a medium load, and a non-lean zone with a high load. When the engine is under low load conditions, a map FLEANPM is selected to obtain an ultra lean air-fuel ratio based on detected intake pressure PM values and engine speed NE. When the engine is under medium load conditions, a map FLEANTA is selected to obtain a medium lean air-fuel ratio based on a detected throttle opening values TA and engine speed NE. When the engine is under high load, conditions the lean correction control is canceled, thereby obtaining a theoretical air-fuel ratio or an air-fuel ratio smaller than the theoretical air-fuel ratio, and when the engine remains at a point in the medium lean zone, the air-fuel ratio decreases gradually toward the theoretical air-fuel ratio as time elapses.