In an oxygen concentration cell having metal electrodes formed on both inner and outer wall surfaces of its solid electrolyte vessel and a solid pole as a reference oxygen partial pressure source made of a metal alone or a mixture of a metal and an oxide of the metal filled in said vessel, a method of activating said oxygen concentration cell by heat-treating the cell so as to diffuse the metal of said reference oxygen partial pressure source into said metal electrode on the inner wall or into said solid electrolyte through the metal on the inner wall surface and the oxygen concentration cell produced by this method.