To eliminate a fear of a heat shield plate being deformed, to facilitate maintenance, and to reduce cost.

A heat shield plate 26 made of stainless is contained in an air distribution casing 21 made of stainless steel having a surface positioned facing a furnace wall 9 and being released, Fall preventing plates 29 and 30 made of stainless steel are fixed to prevent a fall of the heat shield 26. An air duct 7 is arranged in a manner to feed air 8 to the air distribution casing 21. The heat shield plate 26 is provided with a slit 27 through which air 8 supplied to the air distribution casing 21 is supplied in a furnace 1 through a slit 16 formed in a fin 17 of the furnace wall 9. A heat shield rod 28 to shield a radiant heat from the furnace 1 is arranged on the furnace wall 9 side of the heat shield plate 26.