The invention provides a device for detecting an air fuel ratio of an air fuel mixture supplied to the combustion apparatus of an internal combustion engine. The device comprises a sensor cell for developing an electric signal in response to the difference between the concentration of oxygen an exhaust gas and the concentration of oxygen in a reference gas, a controlling means for developing an electric control signal having a polarity determined from an output of the sensor cell, a pump cell for moving oxygen ions in response to an electric control signal received from the controlling means, a control current detecting means for detecting control current flow between the controlling means and the pump cell, an air fuel ratio detecting means for detecting the air fuel ratio from the control current flow, and a stoichiometric air fuel ratio detecting means for detecting the direction of the control current flow so as to determine the stoichiometric air fuel ratio.