To minimize the electric energy by heightening the temperature of discharged gas introduced into a preheating furnace by combusting unburned CO gas inside a melting furnace and heightening the temperature of the gas inside the preheating furnace combusting the unburned CO gas inside the preheating furnace.

Gas analyzers 14A and 14C for measuring the CO content of the discharged gas are arranged to a duct 10 connecting between an A furnace and a B furnace that are arc two units of the arc furnaces and gas analyzers 14B and 14D for measuring the CO content of the discharged gas are arranged to a duct 15A and 15B on the outputting side of a preheating furnace. Scrap 9 is preheated by inputting scrap 9 and carbon blocks 13 as coke into the B furnace where a part of molten steel 8 is remaining and transferring high temperature discharged gas generated by the A furnace to the B furnace through the duct 10. The unburned CO gas generated by the A furnace in melting is measured by the gas analyzer 14A, the blown in quantity of oxygen to the A furnace is controlled based on the measured value and the temperature of the discharged gas introduced into the preheating furnace from the melting furnace is heightened.