An output voltage, which is induced by a solid electrolyte in response to the concentration of oxygen included in exhaust gas from an engine, is compared with a reference voltage, the value of which is different in accordance with an air-fuel ratio reference set in advance for an air-fuel ratio control of the engine, whereby an air-fuel ratio of fuel mixture supplied to the engine is determined on the basis of a signal obtained by simply amplifying a deviation resulted from the comparison by a large amplification factor. Accordingly, the changing range of the signal with respect to air-fuel ratio is expanded remarkably so that the measurement accuracy of an air-fuel ratio sensor is much improved.