To assure the airtightness in furnace cores by preventing the partial wear in the junctions of divided furnace cores and to enable conduction of load regulation so as not to impair the airtightness even if the partial wear occurs.

The heating furnace has furnace tubes 3 and 4 of a multistage structure which are fixed and placed on an anchor block within a furnace casing 7 and are divided and stacked in a perpendicular direction. The furnace executes a heat treatment by introducing prescribed gas into the respective furnace tubes 3 and 4. A plurality of air cylinders 23 are fixed to a top cover 10 of the furnace casing 7 and the furnace tubes 3 and 4 are downwardly pressed by the actuation thereof, by which the airtightness in the divided junctures 3a and 4a of the furnace tubes 3 and 4 is improved. Pushrods 21a and 21b are connected through load cells 22 to the pressing cylinders 23. The heat treatment is performed by actuating the pressing cylinders 23 to press a top cover 8 of the inner furnace tube to the inner furnace tube 3 and pressing a top cover 9 of the outer furnace tube to the outer furnace tube 4 to make the interior of the inner furnace tube 3 and the outer furnace tube 4 airtight.