In a flash smelting method of copper, a carbonaceous material, whose grain size is under 100 um and is in a proportion of 65% or more, and whose grain size is from 44 to 100 .mu.m and is in a proportion of 25% or more, and which has 80% or more of a fixed carbon content, is charged into a reaction shaft of a flash smelting furnace. It is possible to prevent the excessive formation and excessive reduction of Fe.sub.3 O.sub.4 in the slag. Copper loss, erosion of refractories and boiler trouble can be prevented.