PURPOSE: To function effectively a ternary catalyst system by forming a catalyst layer deposited with a catalytically active material on a fine particle carrier consisting essentially of CeO2 and contg. oxide selected from La2O3 and TiO2.

CONSTITUTION: The catalyst layer 5 deposited with the catalytically active material on the fine particle carrier consisting essentially of CeO2 and contg. at least one kind of the oxide selected from La2O3 and TiO2 is formed on a protective layer 4 of an oxygen sensor element for detecting oxygen concn. constituted by providing a reference electrode 2 and a measuring electrode 3 on the surface of an oxygen ion conductive solid electrolyte 1 and providing the porous protective layer on the measuring electrode 3. The element has therefore the substantially high durability at the temp. at which the element is actually used, more particularly not only in a low temp. region but in a high temp. region as well in a vehicle having a non-equil. exhaust gas and a vehicle for which a leaded gasoline is used. The response region of the oxygen sensor element is always maintained near the theoretical air-fuel ratio and therefore the ternary catalyst system functions effectively.