Wastes mainly containing any one or two or more of raw garbage, waste lumber, paper diapers, waste plastics, and organic sludge discharged from the food industry, common houses, etc. are exposed to high-temperature steam of 510 to 900° C. in an oxygen-free state, preferably while being stirred, to be carbonized. Thereby, the amount of wastes is reduced safely and efficiently with no fear of generating harmful substances such as dioxin, and moreover the carbonized wastes can be reused effectively for applications to fuel or the like.