An air-fuel ratio control system for an internal combustion engine includes a pair of cylinder banks, a pair of exhaust passages connected to the cylinder banks, respectively, a common exhaust pipe where the exhaust passages join each other at their downstream ends, a pair of catalytic converters provided in the exhaust passages, respectively, a pair of main air-fuel ratio sensors provided in the exhaust passages upstream of the catalytic converters, respectively, a pair of auxiliary air-fuel ratio sensors provided in the exhaust passages downstream of the catalytic converters, respectively, and a catalytic converter provided in the common exhaust pipe. The system derives an air-fuel ratio feedback control correction value for each of the cylinder banks based on outputs of the auxiliary air-fuel ratio sensors. The system derives the air-fuel ratio feedback control correction values in such a manner as to control the outputs of the auxiliary air-fuel ratio sensors to be in antiphase with each other when the outputs of the auxiliary air-fuel ratio sensors are in phase with each other. This arrangement ensures effective purification of exhaust gases at the catalytic converter provided in the common exhaust pipe.