The authors have developed a non-oxidizing heater system, which can heat up materials such as steel to over 1,273 K under a perfect non-oxidizing atmosphere by the convection heat transfer of a high temperature N-2 gas jet stream. This system, an N-2 jet heater, can heat N-2 gas up to 1,773 K, and can continuously supply the high temperature Np gas using a pair of heaters installed with a regenerative heat exchanger inside the combusion chamber. In this system, the heat of combustion of the fuel can be transfered to the sensible heat of fresh N-2 gas at a thermal efficiency of almost 100%. This system has been applied to the tundish heater of a real continuous easter. As a result, the rate of occurence of surface defects in the quality of steel products is reduced to less than one-fifth of the past level. This paper discusses the mechanism of heat transfer by the high temperature N-2 gas jet stream, presents an outline of the heater system, and describes the effect of its use on steel product quality.