To effectively utilize a slag by reducing a Pb content in the slag to below a prescribed level by supplying a slag containing Zn and Pb components and carbon into a reaction furnace, reacting the slag in a fused state with the carbon and discharging the Pb component made to a vapor state to outside the furnace.

A slag containing Zn and Pb components is obtained by respectively discharging a steel manufacturing dust as a steel manufacturing discharged material from a dust silo 22 and a reduced slag as a steel manufacturing discharged material from a reduced slag silo 24 and blending them. The resultant slag is fed to an oxygen burner 12 by using air as a carrier gas and injected into a high temperature flame in the oxygen burner 12, then fused in a reaction furnace 10. Simultaneously, carbon powder in a vessel 26 is fed to the oxygen burner 12 by using air as a carrier gas and injected into a high temperature flame in the reaction furnace 10 together with the slag containing Zn and Pb components, then subjected to a reaction, thus the Zn and Pb components in the fused slag 28 is discharged as a vapor state and oxidized by air at an exit of the reaction furnace 10, then recovered in a bag dust collector 32 on a downstream side.