The detector includes conductive solid electrolyte by oxygen ion, the first and second electrodes mounted on both sides of the solid electrolyte, a dispersive resistance with hole covering first electrode and a circuit which can supply current between the first and second electrodes. To detect the fuel-air ratio in large range the current between two electrodes is controlled. So the oxygen can be supplied from second electrode to the first electrode through the electrolyte. The moving rate of oxygen is detected when pumping oxygen and the fuel-air ratio is determined. |