To eliminate bending deformation even upon being used for a long period of time at a temperature higher than a specified temperature and prevent the generation of void due to the oxidizing of grain boundary by a method wherein an oxide dispersing reinforced type thermal alloy, containing a specified amount of high melting point metal oxide in ferrite matrix consisting of specified amounts of Cr, Al, Ti and the balance of Fe, is employed.

An oxide dispersing reinforced type heat resistant alloy, containing 0.1-2% of dispersed fine high melting point metal oxide in a ferrite matrix consisting of 10-40wt.% of Cr, 10wt.% or less of Al, 5wt.% or less of Ti and the balance of Fe substantially, is employed for the material of secondary air nozzle for radiant tube. In this case, the oxide dispersing reinforced type heat resistant alloy, employing one kind or two kinds or more of Y2O3, ZrO2, Al2O3 and the like, is employed for the high melting point metal oxide. According to this method, the secondary air nozzle is prominent in resistance to high-temperature oxidizing, resistance to thermal shock, resistance to oxidization and the like while void due to bending deformation will never be generated.