An oxygen sensor is disclosed in which an airtight reference chamber (32) isolated from a surrounding space is formed on one main surface of a solid electrolyte (10), a current is supplied to the solid electrolyte to pump out oxygen from the reference chamber in such a manner that oxygen in the reference chamber is ionized and then passes through the solid electrolyte, means is provided for transferring oxygen from the surrounding space to the reference chamber in such a manner that oxygen in the surrounding space is ionized and then passes through the solid electrolyte, and the oxygen concentration in the surrounding space is detected on the basis of the current supplied to the solid electrolyte.