Nitrogen is fed into a sealed container (18) to expel oxygen (step 1) and, in such a state, the inside temperature of the container (18) is incrementally raised step by a heater built into the container (18). In sequential steps 2 through 5, Water content, chlorine, and high-molecular gases are extracted. Reusable carbon and metal remain in the container (18) in step 6, or when left standing for a prescribed time or more. Gases extracted in steps 2 to 5 can be liquefied for recycling. No carbon dioxide or dioxins are produced because heating is performed in an oxygen-free atmosphere. A furnace is not required because the heater is installed in the container (18), and treatment efficiency is high. A space-saving, energy-efficient, and low-cost treatment system, by which carbon (e.g., inert carbon), metals, and the like can be recovered in a reusable state without producing carbon dioxide or dioxins, is obtained. The obtained inert carbon can be subjected to an activation treatment to continuously mass-produce carbon nanotubes and activated carbon.