To preheat a scrap stably at a high temperature even if the ratio of a combustible gas in an exhaust gas is varied by providing a flow rate controller for a combustion promoting gas, i.e., the air and oxygen, thereby controlling the mean oxygen concentration and the flow rate of the combustion promoting gas being blown into a rotary kiln.

A rotary kiln (preheating furnace) 5 is coupled to the exhaust gas delivery side of a melting furnace 1. The exhaust gas from the melting furnace 1 containing a combustible gas flows in through an opening 7 and fills the preheating furnace 5. A nozzle 6 for blowing a combusting promoting gas is disposed in the vicinity of the opening 7 and a controller controls the flow rate of the combustion promoting gas, i.e., the air and oxygen, being fed to the nozzle 6. A melting material 8 is preheated with the exhaust gas while being fed to the melting furnace 1 as the preheating furnace 5 is turned. The preheated melting material 8 is fed through the opening 7 to the melting furnace 1 where it is melted to produce a molten material 3. At the time of preheating, the combustible gas in the exhaust gas in burnt with the combustion promoting gas being fed through the nozzle thus raising the preheating temperature of the preheated melting material 8.