A thick film type oxygen sensor has been developed. It has the unique structure and principle to achieve the fast light off and a compact configuration. The main part of the uniqueness is the self generation of oxygen from exhaust gas, as the reference. In another words, this does not require fresh air from atmosphere as the conventional thimble type oxygen sensor and no output is shifted down by the change of the reference oxygen partial pressure. The other advantage is monolithic nature of the sensing element and the heater, which makes the sensor start working about in 10 seconds with small wattage. The combination of these two key technologies enables the oxygen sensor to be small and free from location issue.