Abstract: 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.