PURPOSE: To measure a concentration of low temperature oxygen by providing a ceramics type oxygen sensor holding body at a hollow part of cylindrical heat insulating body to form a measuring gas passage between the cylindrical heat insulating body and the holding body while a heat generating body to heat a sensor up to a specified temperature.

CONSTITUTION: An oxygen concentration measuring apparatus 1 is mounted with an oxygen concentration measuring section 3 inserted into a mounting hole disposed in a reactor wall heat insulating body 2a of a heat treating furnace 2. The oxygen concentration measuring section 3 has a measuring gas passage 8 formed between cylindrical heat insulating bodies 4 and 5 and a ceramics type oxygen sensor holder 7 provided at hollow parts of the cylindrical heat insulating bodies 4 and 5 which a module heater heating body 9 is provided to heat the ceramics type oxygen sensor 6. The cylindrical heat insulating body 4 is provided with a gas inflow port 19 for feeding the measuring gas to the measuring gas passage 8 while outflow ports 20 and 20 are provided to discharge the measuring gas after it flows through the measuring gas passage 8.