To provide a heat treatment device capable of suppressing produced gas generated following heat treatment of an object to be heated, and leakage of the so-called sublimates generated by cooling the produced gas.

The heat treatment device has a conversion part 6 for taking in and out a substrate from a heat treatment chamber 12. Air nozzles 65 and 66 capable of jetting clean air are arranged above and below a through hole 44c of a shielding plate 44 composing a boundary portion of the conversion part 6 and the heat treatment chamber 12. When air is jetted from the air nozzles 65 and 66, an air curtain C is formed in the boundary portion of the conversion part 6 and the heat treatment chamber 12. Consequently, leakage of the produced gas and the sublimates generated in the heat treatment chamber 12 to the outside of the heat treatment device 1 is prevented even when the conversion part 6 is opened and closed.