To prevent corrosion or thickness reduction in a center part in the width direction of a furnace wall before it happens.

A required part in the longitudinal direction of each of a plurality of furnace wall pipes 8a among a large number of furnace wall pipes 8 connected by fins 9 to constitute the furnace wall 4c is bent such that the required part projects to a furnace inner side, so as to form a gap 18 between the back side of a bent projection part 17 thereof and the furnace inner side of the straight tube-like furnace wall pipe 8 adjacent to the bent furnace wall pipe. A duct 19 is installed outside the furnace wall 4c such that the duct 19 covers the bent projection part 17. By blowing boundary air 16 led from an air supply part through the duct 19 sideways along the straight tube-like furnace wall pipe 8 from the gap 18, an air film is formed on the furnace inner side of the straight tube-like furnace wall pipe 8. Thereby, contact of hydrogen sulfide gas generated inside a furnace in reducing atmosphere and the furnace wall 4c is prevented.