Abstract: The present invention comprises a processing furnace for oxidizing object to be processed at a high temperature, pressure reducing means for evacuating the interior of the processing furnace, a burning apparatus disposed outside the processing furnace for burning hydrogen gas and oxygen gas to generate water vapor, a water vapor supply pipe interconnecting the burning apparatus and to the processing furnace, and a throttle disposed on the water vapor supply pipe for generating a pressure difference in the water vapor supply pipe between a side of the burning apparatus and a side of the processing furnace. Stable burning is ensured in the burning apparatus, which makes set oxidation under low pressures possible. It is possible that in place of the throttle, atomization means for atomizing pure water, or a boiling water vapor generating unit, or a microwave water vapor generating unit.