To provide a minimum electrical power energy required for melting and refining scrap by a method wherein oxygen for use in igniting CO in a preheating furnace is supplied on the basis of a detected value of CO concentration in discharged gas discharged from above a surface where a moving furnace body is installed.

A furnace lid 4 is installed at the upper part of a furnace A, and a movable moving furnace body 8 is arranged between a furnace main body 1 and the furnace lid 4 and then arc is generated between a furnace bottom electrode 5 placed at the bottom part of the furnace and a movable electrode 6 at the upper part of the furnace. Then, at a furnace B, molten steel 7 is partially left and scrap 11 to be melted is filled in the furnace in advance, hot discharged gas discharged from the furnace A is fed to the furnace B through a duct 10 so as to heat the scrap in advance. An oxygen blowing-in device 9 is mounted at the moving furnace body 8, either oxygen or gas containing oxygen is blown into the furnace to react with CO gas in a pre-heating furnace. As a blowing-in amount of oxygen, a CO concentration in the discharged gas discharged from the preheating furnace B is measured by a device 14 and then an amount of oxygen required for performing a complete combustion of CO gas is blown into the furnace.