To provide a heat treatment furnace the heat insulation material of which can be cooled inexpensively and effectively.

The heat treatment furnace is provided with a pressure vessel 1, the heat insulation material 2 which is installed in the vessel 1 and forms a heating space S, and evacuating systems 8 and 9 which evacuate the inside of the vessel 1 to a vacuum. The furnace is also provided with a gas introducing system 3 which introduces a gas into the vessel 1. The main body 21 of the heat insulation material 2 is prepared by laminating multiple heat insulation elements A each of which is constituted by superposing graphite felt a1 upon graphite foil a2 upon another.