To provide a method for treating waste that contains an alkaline component by combustion in a smelting furnace, wherein the deposition of flue cinder in exhaust gas treatment equipment etc. is suppressed.

In the method for treating the waste that contains the alkaline component by combustion in the smelting furnace, and returning boiler ash collected from a boiler of the smelting furnace and/or Cottrell ash collected from an electric dust collector to the smelting furnace for repetitive treatment, the collected ash is made into water slurry before returned to the smelting furnace so that the alkaline component is eluted, and leaching residues obtained by solid-liquid separating the water slurry are returned to the smelting furnace so that the deposition of flue cinder is suppressed.