In the operation, a flux mainly composed of silica ore and a non-ferrous metal-ore raw-material are charged into a smelting furnace via a conveying system. In order to increase the production amount of the metal, the flux is conveyed and treated through a first system, in which the flux is crushed in a ball mill and dried in the bail mill, and the crushed and dried flux is conveyed directly before the smelting furnace. The non-ferrous metal ore is treated and conveyed through a second system, in which it is dried with a drier and then conveyed directly before the smelting furnace. In the drier of copper concentrate, the flux is not dried at all.