PURPOSE: To reduce the thickness of furnace wall so as to make the weight of the furnace smaller, by lining the steel sheet of furnace wall with fibrous heat insulating material of specific thickness, and by attaching ceramic boards of specific thickness, coated with glaze consisting essentially of silicic acid, on the surface of the insulating material.

CONSTITUTION: A heat treating furance 1 in which a reducing gas is used as an atmosphere is constituted of a ceiling wall 2, side walls 3, and a bottom wall 4. The steel sheet 11 of the ceiling wall 2 and the side wall 3 is lined with fibrous heat insulating material 12 such as ceramic wool, slag wool 75W300mm in thickness, and the lining is fastened by pins 13. To the surfaces of the insulating materials are attached ceramic boards 14W25mm in thickness coated with glaze consisting essentially of silicic acid. Compressed alumina silicate fiber is used as the ceramic boards. Hence, this method allows to reduce the wall thickness to 1/2 that of conventional wall, and to reduce the weight of furnace to 1/5W1/10 that of conventional furnace.