To effectively utilize as a heat generating source and slag foaming at steel-making time by intermittently or continuously charging combustible solid materials into a furnace at the high temp. stage in the operating time of the electric furnace, burning and also, utilizing gas generated with the combustion to the slag foaming.

A combustible solid material formed as grain state or small block state, etc., obtd. from waste material is intermittently or continuously charged into the electric furnace 1 at the high temp. stage in the operating time of the electric furnace, in which the exhaust gas temp. at the outlet 2 of the electric furnace 1 desirably reaches ≥800°C on and after the point of time when the slag and molten steel produce in the electric furnace 1. In this way, the slag in the molten stage and the molten steel are heated. Further, since the combustible solid materials are formed as grain state or small block state, etc., these materials are not quickly burnt and thus, the gas generated with the combustion can stably be utilized to the slag foaming. The charging quantity of the combustible solid material is suitable to 10-100 kg/min in the general electric furnace 1 having 100 t/ch capacity.