A feedback control system using an oxygen-sensitive air/fuel ratio sensor which is disposed in induction passage for an IC engine downstream of a fuel supply device with the provision of an ignition means to burn a fractional portion of an air-fuel mixture in the induction passage such that the oxygen-sensitive sensor is exposed to a resultant combustion gas. The sensor is of the concentration cell type having a layer of solid electrolyte such as zirconia provided with two electrode layers, and constant DC current is supplied to this sensor to cause migration of oxygen ions through the solid electrolyte layer, whereby this sensor exhibits a slope output characteristic and can detect air/fuel ratios either above or below a stoichiometric ratio or exhibits an on-off characteristic and can detect the stoichiometric ratio depending on the intensity and flow direction of the current. Owing to reduced overall length of the closed-loop, this control system can accomplish correction of a deviated mixing ratio in a shortened period of time.