A system for feedback control of air/fuel ratio in an internal combustion engine operated with a nonstoichiometric air-fuel mixture. The control system includes two oxygen-sensitive air/fuel ratio sensors which are of the concentration cell type having a solid electrolyte layer provided with two electrode layers and both disposed in an exhaust passage substantially at the same section. The first sensor is supplied with a constant DC current to cause migration of oxygen ions through the solid electrolyte layer and exhibits a slope output characteristic when a lean mixture or a rich mixture is supplied to the engine, depending on the direction of flow of the current. The second sensor exhibits on-off type output characteristic and can discriminate between a lean mixture and a rich mixture. A control circuit to produce a fuel feed rate control signal based on the output of the first sensor includes a discriminating means for ascertaining whether the output of the first sensor is truly attributed to the slope output characteristic with reference to the output of the second sensor.