PURPOSE: To preheat material to be melted economically by utilizing the sensible heat of high-temperature waste gas effectively by a method in which a gas for heating material to be melted is heated by high-temperature waste gas from electric oven and then the material is heated by means of convectional heat transmission by the heated gas.

CONSTITUTION: High-temperature waste gas discharged from the electric oven 1 is introduced into the upper part of the heat exchanger 3, where the cooled waste gas by heat exchange is directed to the lower part of the cooler 5 to cool it with atomized cooling water and then released through the dust collector 6 and then the stack 8 by means of the inductive air blower 7. The heated air through heat exchange with the waste gas in the heat exchanger 3 is directed to any of the preheaters 10 and 11, where it is brought into contact with scraps inside the oven 10, for example, to heat it by means of convectional heat transmission. Then, the air cooled through heat exchange with the scraps is returned to the heat exchanger 3 through the dust collector 13 by the blower 14.