An air-fuel ratio controlling apparatus includes an internal pressure detector for detecting an internal pressure of a combustion chamber of the engine. The apparatus estimates a motoring pressure of the engine and determines a start-of-combustion time, a time point when a difference between the internal pressure and the motoring pressure exceeds a predetermined value in a compression stroke and a combustion stroke of the engine. Firing delay for each cylinder is calculated from as a duration from sparking to the start-of-combustion time. Air-fuel ratio of each cylinder is estimated based on the firing delay and fuel injection amount for each cylinder is calculated to make the air-fuel ratio of plural cylinders uniform.