To prevent the trouble in an equipment caused by the combustion heat of unburnt gas in waste gas and to simultaneously improve the preheating efficiency of an iron source, at the time of preheating the iron source in a preheating vessel with exhaust gas from an electric furnace.

Plural nozzles 14, 15, 16, 17, 18 are arranged in the flowing direction of the exhaust gas in the preheating vessel 7, and oxygen-containing gas is blown into the preheating vessel from these nozzles to preheat the iron source 10 while burning the unburnt gas in the exhaust gas. At this time, the combustion of the unburnt gas is accurately controlled by individually controlling the blowing quantity of the oxygen-containing gas and the blowing angle of the nozzle for each nozzle, thereby the heat conductive efficiency is improved.