A system for treating a treatable material containing a noxious component, which comprises a mixer for mixing a treatment agent containing alkali material with the treatable material to form a mixture, at least one first heat treating furnace for producing a low oxygen concentration atmosphere, a first heating device located outside the first furnace to heat it at a first temperature at which the treatable material is decomposed to generate a substance containing the noxious component, at least one separate second heat treating furnace, and a second heating device located outside the second furnace for heating the treatable material residue at a second temperature at which carbonization of the treatable material residue takes place.