An air-fuel ratio sensor comprising a limiting electric current type oxygen sensor portion, having a sequential stacking on a porous substrate of a first electrode with high gas permeability, a solid electrolytic thin film having a specific crystal orientation, a thickness of 0.1 to 30 .mu.m, and good crystallinity. The limiting electric current type oxygen sensor portion and second electrode with a high gas permeability, is arranged and connected in parallel with a resistive type oxygen sensor portion having interdigital electrodes formed on one or both major surfaces of an oxide semiconductor thin film, the resistance of which changes in accordance with oxygen partial pressure. The air-fuel ratio sensor also has a sensor heater formed on one surface of the porous substrate around the sensor portions. The overall structure of the sensor is coated with a porous ceramic coating to which a catalyst is added.