To improve fixing of a heater principally comprising MoSi2 and the structure of a furnace and to facilitate maintenance/inspection of the heater or the furnace wall or replacement of a damaged heat insulating furnace wall (unit) including the heater.

The cylindrical furnace comprising vertically separable heat insulating furnace wall in multistage is provided, on the inner surface of heat insulating furnace wall at each stage, with a heater independently. A level difference is provided in the thickness direction of furnace wall at the joint of respective stages of heat insulating furnace wall and an heat insulating spacer is provided at the level difference part.