Abstract This paper seeks for cause and countermeasure way of fires of huge amount of rubble which were produced after 2011 great earthquake of Japan. In 2011, we experienced many fires caused with rubbles which were produced from destroyed houses by the Great earthquake and tsunami in March 11, 2011, in Japan. Rubble includes various organic materials, and sometimes causes fires, which is used for fuel of power plants, or energy sources. It is very difficult to extinguish fire of such biomass fuel made from rubble in outdoor storage facilities. Here current studies for safety handling of these materials and proposed an evaluation method, is introduced, which is to use high sensitive calorimeters. And cause investigation work and countermeasure method, conducted by author is introduced. Our results regarding cause of fires are: initial of heat generation and fire are mostly by fermentation, and then oxidation process started after micro organism dead by high temperature, up to about 60°C. High sensitive calorimeters can detect small heat generation between room temperature and 80°C, due to fermentation or other causes. This heat generation sometimes initiated a real fire even outdoor, and produced some combustible gas. With understanding this process we recommend countermeasure way against such fires, to release heat from the pile, and prevent air entrainment into the pile to stop fermentation.