In an A/F ratio sensor of mixture supplied for an internal combustion engine, which has a detector for detecting the concentration of residual oxygen in exhaust gas and determines the A/F ratio by retrieving an air excess ratio table with an output of the detector, there is further provided an additional air introducing device, which introduces a predetermined quantity of air into the engine for a certain period of time, under the condition that the velocity of air flowing through a throttle valve is equal to the sonic velocity. The oxygen concentration detector is calibrated by the comparison of the outputs thereof before and after introduction of the additional air. According to this invention, the A/F ratio of the mixture can be detected accurately without the influence of the aged deterioration of the oxygen concentration.