In a method of operating a copper smelting furnace, wherein a ferrous substance containing more than 80 wt. % metallic iron having a specific gravity of 3.0-8.0 and particle diameter of 0.3-15 mm is added to copper smelting slag containing Fe having an oxidation-reduction number of 3+ and to the Fe3O4 in the intermediate layer, thereby deoxidizing the Fe3O4 to FeO, the method reduces the Fe3O4 within the slag layer and the Fe3O4 generated in the intermediate layer between the slag layer and the matte layer. So that their viscosity is reduced and separation rate is increased, thus increasing the yield rate of useful metal, and the problems that originate in the intermediate layer are eliminated.