To enhance combustibility of an exhaust gas by arranging an assist burner for burning combustible gas in the exhaust gas from a scrap melting furnace at a specific angle at a bottom of a rotary kiln type scrap preheating furnace in the scrap heater continuously provided at the inclined furnace at an upper portion of the melting furnace.

One end of an inclined rotary kiln type scrap preheating furnace 2 is connected to an upper portion of a scarp melting furnace 1, and a lower portion of a shaft type second preheating furnace 3 extended longitudinally is continuously provided with its another end. Scrap charged from the upper portion of the furnace 3 is preheated by the furnace 3, then again preheated in the case of passing the furnace 2, and thereafter charged from above the furnace 1 and melted. CO, H2 gas or the like in the exhaust gas 4 generated from the furnace 1 is burned by an assist burner 5. The burner 5 is arranged at an angle crossing with a bottom of the furnace 2 downward to a central axis of the burner, and combustibility of the exhaust gas is improved.