0 An air-fuel ratio sensor installed in exhaust gas emitted by an internal combustion engine for detecting from the exhaust composition on the air-fuel ratio in the engine becoming equal to a stoichiometric air-fuel ratio. The sensor comprises a solid electrolytic layer of zirconia (10), a pair of electrodes (20, 30) formed on both sides of the electrolytic layer, and a cover (70) provided over the electrolytic layer to enclose one of the electrodes, with a diffusion room formed between the cover and the electrolytic layer and with a gas diffusion hole (5) formed to communicate the diffusion room with the outside of the sensor. The electrode (20) in the diffusion room works as a positive electrode, while another electrode (30) works as a negative electrode to conduct a current (lp) between these electrodes so as to control the oxygen concentration in the diffusion room as a reference oxygen concentration. The fact that the air-fuel ratio has become equal to the stoichiometric air-fuel ratio is detected from a voltage (Eo) appearing between the electrodes.