In order to realize the treatment for an organic material anaerobically, which contains a big amount of a solid matter, and in order to deal with the organic material at a high decomposition rate and at a lower cost, the method and apparatus provides: the steps of: solubilizing the solid organic material in the organic material by alkali treatment to obtain the soluble organic material; applying acid fermentation to the organic material containing the soluble organic material, obtained in the solubilization step, by anaerobic bacteria; separating the material obtained in the acid fermentation step into the solid component and into the liquid component; applying methane fermentation to the liquid component obtained in the separation step for obtaining the digestive gas; introducing the digestive gas to the acid fermentation step, introducing the generating ammonia in the acid fermentation step into the digestive gas; and taking out the ammonia, together with the digestive gas from the acid fermentation system, thus removing the ammonia; and recycling the solid component obtained in the separation step to the solubilization step.