PURPOSE: To detect the air/fuel ratio in a wide range, by providing an oxygen concentration cell element through a fine interval from an oxygen pump element and a metal oxide semiconductor made of titania attached thereto.

CONSTITUTION: Porous electrodes 4 and 5 are provided on both sides of a planar oxygen ion conductive solid electrolyte 3 while an air chamber (b) is formed on one side thereof to make an oxygen pump element 6 by being exposed to the atmospheric air. An oxygen concentration cell element 13 in which porous electrodes 11 and 12 are formed on both sides of an identical planar oxygen ion conductive solid electrolyte 10 is provided through a fine interval from the element. A metal oxide semiconductor 17 made of titania formed on an electrical insulator 16 having an opening (d) is built as attached to a porous electrode 12 of the oxygen concentration cell element 13. To detect the air/fuel ratio, the resistance value of the titania 17 is used to determine whether the fuel is in the over rich range or lean range and then, the pump current Ip of the oxygen pump element 6 is measured.