A detection membrane is joined to a first surface of an electrolyte membrane. After the detection membrane is hydrogenated, oxygen is supplied to a space facing a second surface of the electrolyte membrane. If the electrolyte membrane has a defect, oxygen leaks to the first surface, resulting in a change in resistance of the detection membrane owing to dehydrogenation of the detection membrane. The defect is recognized by this change. An air electrode is joined to the second surface, and an electric circuit is connected between the detection membrane and the air electrode. After hydrogenating the detection membrane and ionizing oxygen supplied to a space facing the air electrode, oxygen ions permeate through the electrolyte membrane and dehydrogenate the detection membrane. Uniformity of the oxygen ion conductivity is examined by measuring resistance of the detection membrane, which varies depending on the amount of oxygen ions, for each region.