NOVELTY - A processing tank (1) carries out anaerobic fermentation of the organic waste, and a processing tank (2) carries out microbial treatment of the fermented liquor from the processing tank (1). The processing tank (2) consists of several tanks (21-24) connected in series. The most upstream tank is comprised such that it is switchable to both aerobic-treatment conditions and anaerobic-treatment conditions.    USE - Organic-waste processing system for carrying out fermentation process of organic waste such as livestock excrement. Can also be used for processing domestic organic refuse, sewage-treatment sludge, and industrial waste organic refuse such as green farm waste material, agriculture-and-fishery waste material, food-processing waste material, etc.    ADVANTAGE - The load of organic-waste processing carried out in the first processing tank can be distributed, since the generation amount of organic waste is varied, so that the processing tank can be designed small and its cost can be reduced, while improving the performance efficiency of system and securing the production amount of biogas.    DETAILED DESCRIPTION - An anaerobic treatment is carried out in the tank of the highest flow, when the residence time of the organic waste in the processing tank (1) becomes below a setting value by the variation of the generation amount in the generation source of the organic waste. A biogas utilization apparatus (7) is comprised such that the biogas which carried out anaerobic treatment and generated in the processing tank (2) is again sent to the biogas utilization apparatus. The processing tank (2) is comprised as a serial multi-tank structure, and an air-supply unit is provided to feed air into the processing tank (2).    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the organic-waste processing system.    Processing tanks (1,2)    Denitrifying apparatus (3)    De-sulfuring apparatus (4)    Biogas utilization apparatus (7)    Livestock barn (11)    Tanks (21-24)