According to our experimental results, the following oxygen probe recommended, that is, the magnesia stabilized zirconia was employed as a solid electrolyte and Fe.FexO solid elect- rode was used as a reference electrode. Using such an oxygen probe, we tried to measure the oxygen potentials in molten metals, mattes and gases in the following fields of non-ferrous metal industry, molten copper in the side launder before casting, molten crude lead in the forehearth of ISP blast furnace, molten copper mattes in the forehearth of blast furnace and in the settler of flash smelting furnace, gas in the settler of copper flash smelting furnace and flue gas in the electric furnace for ferro-silicon. In the past ten years or so, many experi- mental results in the field of fundamental studies have confirmed that the equilibrium oxygen potential could be measured conveniently and accurately by an electrochemical method using a zirconia solid electrolyte. Recently, such an electrochemical technique has been also applied to a device for physicochemical invest- igation or process control in the non-ferrous metal industry. Such a device consists of an oxygen con- centration cell with stabilized zirconia, and it is commonly called an "oxygen probe" or "oxygen sensor".