An air/fuel ratio control system is provided for an internal combustion engine. The system includes first and second oxygen density sensors, an air/fuel ratio control device and a standard-value changing device. The first oxygen density sensor is arranged on an upstream side of a catalytic converter, while the second oxygen density sensor is provided either inside or on a downstream side of the catalytic converter. The air/fuel control device controls the air/fuel ratio of the internal combustion engine on the basis of results of comparison between a detection value from one of the first and second oxygen density sensors and a predetermined standard value. The standard-value changing device changes the standard value on the basis of outputs from the first and second oxygen density sensors.