To solve environmental problems and treatment problems of incinerator ash caused by incineration by using a carbonization apparatus having a closed double structure and carbonizing organic matters, e.g. garbage, paper products, pruned trees, barks, etc.

Any kinds of organic matters, e.g. garbage, paper products, pruned trees, barks, etc., which are incinerated or partly used for landfilling are carbonized by powdered activated carbon and soil amending agents, pulverized water treating agents and air cleaning agents and dehumidifying agents usable under the floor, and solidified solid fuels using a carbonization apparatus having a closed type double structure and of which charcoal yielding efficiency is remarkably heightened and carbonization time is greatly shortened as compared with a charcoal kiln. Consequently, environment problems and incinerator ash treatment problems caused by incineration are braked and the decrease of landfilling sites is also suppressed.