NOVELTY - Biomass is dried and separated into thermal decomposition gas and solid carbonized material. The thermal decomposition gas is converted into low-boiling-point gas by modification reaction using a catalyst which reacts at 400-900degreesC. The reformed gas is cooled and purified after modification, and moisture content, heavy metals, and corrosion components in gas are removed. The purified gas is then introduced to the thermal decomposition process and drying process, and utilized as a heat source.    USE - For carbonizing biomass, such as sewage sludge, human-waste sludge, livestock excrement, digested sludge, dewatered sludge, foodstuffs waste materials, and their composted materials, and using obtained solid carbonized material as compost raw material, fuel for coal-burning thermal power stations, or melting fuel substitute of shaft-furnace type waste-material melting furnace (all claimed).    ADVANTAGE - The method decreases the input of external energy even when biomass of high moisture content is used. The heat loss by exhaust gas is reduced. Modification of tar in the thermal decomposition gas prevents failure by tar obstruction. Since heavy metals and corrosion components are removed, the durability of the apparatus is improved and hazardous components in waste gas are reduced.    DETAILED DESCRIPTION - Biomass is dried and separated into thermal decomposition gas and solid carbonized material by indirect hearting at 400-900degreesC in an air-interrupted state. The thermal decomposition gas containing high-boiling-point liquid gas which liquefies at normal temperature and pressure, is converted into low-boiling-point gas, such as hydrogen, carbon monoxide, and methane, by modification reaction using a catalyst which reacts at 400-900degreesC. The reformed gas is cooled and purified after modification, and moisture content, heavy metals, and corrosion components in gas are removed. The purified gas is then introduced to the thermal decomposition process and drying process, and utilized as a heat source.    DESCRIPTION OF DRAWING(S) - The figure shows the block diagram of the carbonization apparatus. (Drawing includes non-English language text)    Washing tank (1)    Dehydrator (2)    Drying thermal decomposition furnace (6)    Catalytic-reaction device (7)    Dust collector (11)