To provide a heat treatment furnace for easily supplying an atmospheric gas into a furnace body at a low cost without generating dew condensation and capable of saving energy.

This heat treatment furnace, wherein the atmospheric gas is supplied from the outside to the furnace body 2 for heat treatment, is provided with a heat insulating chamber 13, with inner temperature kept higher than the dew point of the atmospheric gas by of the heat transmitted from the furnace body 2, and an atmospheric gas supply duct 4 for supplying the atmospheric gas into the furnace body 2 is led into the furnace body and arranged through the inside of the heat insulating chamber.