PURPOSE: To measure the oxygen content of a gas by maintaining a tube which can conduct oxygen ions at a controlled temperature and measuring the ambient temperature of the tube with a temperature measuring means connected to an oxygen quantity displaying means, and then, automatically correcting the deviation of oxygen content measurements based on the temperature difference between the ambient temperature and controlled temperature.

CONSTITUTION: The housing 14 of an oxygen content measuring instrument is passed through 16 a boiler wall 16 and the front side of the housing 14 is surrounded with a filter 20 and a flame arrester 22. A tube-like electrochemical battery 24 is supported in the housing 14 and closed and opened ends 25 and 27 are provided. Then the battery 24 is attached to a seal section 32 which separates a flue gas circulating area from an atmosphere enclosing area 36. The outside of the battery 24 is exposed to a flue gas and the inside of the battery 24 is exposed to the atmosphere. In addition, a heater device 38 having lead wires 40 is provided and the inside of the battery is maintained at a fixed controlled temperature equal to the ambient temperature in a boiler room, and then, the deviation of oxygen content measurements is automatically corrected.