To keep the interior of a furnace in specified atmosphere in its early stages, and shorten the time until its reach to stationary operation, by providing an electric furnace with space between a furnace shell and an outermost heat insulating layer, so as to perform the exhaust from the of the outermost shell of the furnace shell.

For a batch system of electric furnace, the outside section of the outermost heat insulating layer 3 is a vacant space 1. A ceramic molded item is set to a furnace room, and exhaust and replacement within a furnace are performed, using exhaust and replacement holes 4 and 5, and then it is baked at 1350°C. In this case, not only the time of initial air exhaust and gas replacement within the furnace is shortened, but also the temperature rise and cooling time is shortened. This is because the propagation (convection and leveling of heat conduction by it) to each part within the furnace of replaced atmospheric gas is made efficient. Hereby, the operation cycle time in total is shortened.