To melt scarp with high thermal efficiency by introducing a part of exhaust gas on the outlet side of a rotary kiln to a preheating vessel when the heating value of exhaust gas from a melting furnace is higher than a specified level thereby performing preheating of scrap and sub-materials simultaneously with decarbonation.

A bypass duct 9 is provided at the coupling part 8 of a rotary kiln and a shaft furnace 3 and coupled with a preheating vessel 10 filled with sub-materials 11 for steel making. Air flow regulation dampers 13, 15 are disposed, respectively, in the ducts 12, 14 on the outlet side of the shaft furnace 3 and the preheating vessel. When the heating value of exhaust gas from a melting furnace is higher than a specified level, the damper 15 is opened to introduce a part of exhaust gas on the outlet side of the rotary kiln 2 to the preheating vessel 10 filled with sub-materials 11 for steel making containing a carbonate, e.g. limestone, dolomite, or the like, and preheating of scrap and the sub-materials 11 for steel making is performed simultaneously with decarbonation.