To provide a heat treatment facility for performing intended heat treatment with the surface of a treated object in the anti-oxidation state, and its method.

In a heat treatment device, a heating furnace 10 and a salt bath chamber 20 are partitioned by a door 100 and brought into contact with each other. Atomospheric gas is intensively introduced to the heating furnace, and a gas passage 110 is provided in the door between the heating furnace and the salt bath chamber so as to make the atomospheric gas introduced to the heating furnace flow into the salt bath chamber. The salt bath chamber includes a sealed structure and is shut off from the outside air. The atomospheric gas made to flow in from the heating furnace makes the atmosphere in the salt bath chamber become anti-oxidation atmosphere, and surplus gas is burned and exhausted through an exhaust port 300. The internal pressure of the heating furnace at this time is maintained at a higher level than that of the internal pressure of the salt bath chamber, to prevent the atomospheric gas from flowing back to the heating furnace by the salt bath chamber. A pressure relief valve 400 is provided in the upper part of the salt bath chamber and is opened/closed when the internal pressure of the salt bath chamber becomes higher than that of the heating furnace. The heat treatment device is further provided with a safety circuit for performing super purge during rapid decline in the furnace internal pressure generated when the door is opened/closed or when a treated object is immersed in the salt bath chamber.