A detecting element is configured such that a second electrode is disposed on one side of a plate-like first electrode with a first solid electrolyte layer positioned therebetween and a third electrode is disposed on the other side of the first electrode with a second solid electrolyte layer positioned therebetween, thereby detecting migration of oxygen ions between the first electrode and the second electrode and migration of oxygen ions between the first electrode and the third electrode. Since the second electrode is disposed on one side of the plate-like first electrode with the first solid electrolyte layer positioned therebetween and the third electrode is disposed on the other side of the first electrode with the second solid electrolyte layer positioned therebetween, both planes of the detecting element functions as detecting planes and, to thus extend the directivity of detection in two directions. Since either the second electrode or the third electrode faces to the flow of exhaust gas, the inventive oxygen sensor can be brought in contact with exhaust gas earlier than a prior art oxygen sensor does. Accordingly, the response time of the inventive oxygen sensor becomes shorter than that of the prior art oxygen sensor.