To provide a heat treatment furnace, preventing the interior of a heat insulating material casing from being saturated or almost saturated with vapor in a process of heating in reduced pressure.

This heat treatment furnace 1 includes a vacuum chamber 2 and the heat insulating material casing 3 disposed in the vacuum chamber 2 and having a heat treatment part 4 and a heat source 5 in the interior thereof, wherein a vent hole 6 communicating the interior of the heat insulating material casing 3 to the outside is formed in the heat insulating material casing 3, and a heat insulating member 7 having a heat insulating surface 8 disposed at a predetermined space from the heat insulating material casing 3 is provided on the axis 9 in the ventilating direction of the vent hole 6.