An oxygen sensor is proposed which is represented by an electrochemical cell: metal | oxide scale | sensing electrode, where the metal, its oxide scale, and sensing electrode work as reference electrode, electrolyte, and sample electrode, respectively. Here the oxide scale is required to be an oxide-ion conductor, and the sensing electrode is not to be reactive with the oxygen. It is expected that the electrolyte is self-restorative because it can be reformed by high-temperature oxidation. The electromotive force (EMF) measurements were carried out at 873 K with cells using zirconium as the metal electrode and Pt as the sensing electrode. At po(2) = 1-10-4 atm, the EMF vs. log po(2) plot lies on a straight line and its gradient is 2.303 RT/4F, suggesting unity of the oxide-ion transference number at the surface of the scale. The EMF steeply decreases with decreasing po(2) at po(2) < 10-4 atm, which cannot be explained by the increase in the electronic conductivity. The oxidation behaviors showed linear oxidation. Assuming repetition which constituted of parabolic oxide film growth until a certain thickness and its crack formation, the linear rate constants were described as a function of the oxygen partial pressure. It was considered that the steep decrease in EMF is caused by the change of the rate-determining process to form the scale.