The purpose of the present invention is to provide a heat treatment furnace for quenching that makes it possible to forgo the used of a converted gas and suitably prevent the appearance of a modified layer resulting from decarburization or the like in the surface of a treated object such as steel. A heat treatment furnace (10) according to one embodiment of the present invention is provided with: a gas supply unit (26) configured so as to supply a neutral gas or an inactive gas to a quenching heating chamber (12); and an internal structure such as a muffle (34) in the quenching heating chamber that is at least partially made from a graphite-based material.
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