A novel oxygen sensor was proposed. The sensor, an electrochemical cell, was composed of a metal as reference electrode, its oxide scale as electrolyte and Pt or other adequate materials as sample electrode. It is expected that the electrolyte is self-restorative because it can be restored by high temperature oxidation. The emf measurements were carried out at 873 K in cells using zirconium. At PO2 = 1 ∼ 10−3 atm, the emf vs. PO2 plot lies on a straight line and its gradient is 2.303RT/4F, suggesting tion = 1 at the surface of the scale. The emf steeply decreases with decreasing PO2 at PO2 < 10−3 atm which can not be explained by the increase in the electronic conductivity and is explained by a gas laminar film phase at the surface of the sample electrode.