NOVELTY - Method of reducing volume of decontaminated waste material comprising organic substance, involves performing composting process which ferments the decontaminated waste material, and incinerating the waste material passed through the composting process. The composting process includes deposition process, sheet covering process, inhalation process, stir and mixture process, state observation process and primary fermentation determination process.    USE - The method is useful for reducing volume of decontaminated waste material comprising organic substance (claimed), e.g. decontaminated grass and trees. Can also be used in nuclear power station.    ADVANTAGE - The method rapidly and cost-effectively reduces large volume of decontaminated waste material and thus is environmentally-friendly. The incineration-processed decontaminated waste material can be stored without taking the countermeasure against a fire during storage.    DETAILED DESCRIPTION - Method of reducing volume of decontaminated waste material comprising organic substance, involves performing composting process which ferments the decontaminated waste material, and incinerating the waste material passed through the composting process. The composting process includes deposition process, sheet covering process, inhalation process, stir and mixture process, state observation process and primary fermentation determination process. The decontaminated waste material is piled up and deposited on the compost yard which constructs the water barrier sheet at the deposition process. In sheet covering process, the decontaminated waste material deposited at the deposition process is covered with a waterproof sheet and a covering is formed. At the inhalation process, with the inlet pipe construct in the covering, the gas inside a covering is sucked and it discharges to the outside of the covering. At the stir and mixture process, the decontaminated waste material deposited on the compost yard is stir-mixed at any time and the state in the covering is measured at the state observation process. In the primary fermentation determination process, the decontaminated waste material fermented primarily is determined based on the result measured at the state observation process.