PURPOSE: To make it possible to measure the concn. of oxygen in atmospheric gas by such a simple structure that electrodes are provided to both sides of an oxygen ion conductive solid electrolyte plate and one electrode side is covered with an oxygen gas shielding body through a gap part.

CONSTITUTION: Constant voltage is applied between lead wires 4a, 5a from electrodes 4, 5 from a power source 9. In this case, when the concn. of oxygen in the exhaust gas of an internal combustion engine being an atmosphere to be measured of an oxygen sensor 1 is low, the quantity of a current becomes constant at relatively low voltage. On the other hand, when the concn. of oxygen in the exhaust gas is high, the quantity of a current is made constant at relatively high voltage. Therefore, if certain adequate voltage is selected, the concn. of oxygen in the exhaust gas can be measured from the quantity of a current flowed at said voltage. When the electrode 4 is used as a minus side and the electrode 5 is used as a pluse side while voltage is applied therebetween, a solid electrolyte plate 3 is acted as a pumping-up oxygen pump and has action for exhausting only oxygen of a gap part 7 to the side of the electrode 5 through the electrolyte plate 3. An oxygen concn., an oxygen amount and the quantity of a current are proportional to each other.