A system for feedback control of air/fuel ratio in an IC engine, utilizing a recently developed oxygen-sensitive device which is provided with a heater and disposed in exhaust gas to provide a feedback signal. This device has a porous solid electrolyte layer with a measurement electrode layer on one side and a reference electrode on the other side facing a substrate. There is a circuit to supply a heating current to the heater and force a DC current to flow in the solid electrolyte layer to cause migration of oxygen ions through the solid electrolyte toward the reference electrode to thereby establish a reference oxygen partial pressure on the reference side of the solid electrolyte layer. To prevent lowering of this oxygen partial pressure while the engine is operated under a high-load condition and the feedback control is discontinued to feed the engine with a fuel-enriched mixture, the control system includes operating condition sensor means and switching means to interrupt the supply of the current to the heater in response to a command signal from the sensor means and, optionally, current regulating means to increase the intensity of current flowing in the solid electrolyte also in response to the command signal.