1. An air-fuel ratio control system for an internal combustion engine comprising: a catalytic device to be installed in an exhaust pipe of an internal combustion engine; a first air-fuel ratio sensor to be installed in the exhaust pipe upstream of said catalytic device to measure an air-fuel ratio of an exhaust gas flowing through the exhaust pipe; a second air-fuel ratio sensor to be installed in the exhaust pipe downstream of said catalytic device to measure an air-fuel ratio of the exhaust gas flowing through the exhaust pipe; and a feedback circuit working to determine a preselected controlled parameter based on the air-fuel ratios, as measured by said first and second air-fuel ratio sensors, and perform air-fuel ratio feedback control.