An air-fuel ratio control apparatus includes: a determining part for determining a target purge ratio in accordance with operating conditions of an engine; a purge control part for controlling a flow rate of evaporated fuel, supplied from an evaporated fuel purge system into an intake passage of the engine, by actuating a purge control valve based on the target purge ratio, and for storing an evaporated fuel flow rate of the purge control valve; a fuel injection control part for generating a drive signal in accordance with a fuel injection time, and for controlling an air-fuel ratio of an air-fuel mixture to the intake passage by actuating a fuel injection valve in accordance with the drive signal; an estimating part for estimating the ratio of an evaporated fuel flow rate to an intake air flow rate based on sensed operating conditions of the engine and on the stored evaporated fuel flow rate of the purge control part; and a fuel injection time determining part for determining a fuel injection time based on the estimated ratio, and for supplying the fuel injection time to the fuel injection control part.