NOVELTY - The digester has an agitating blade (90) that is installed in outer periphery of an inner container (30). Two supporting shafts (50a,50b) are installed in left and right sides of an outer container (10), to support an acid fermenter which performs acid fermentation process of organic waste. The acid fermenter is rotated by delivering rotation power to supporting shaft, so as to mix acid fermented organic waste. The acid fermented organic waste is flowed out into outer container through an outlet (35).    USE - Horizontal anaerobic digester for processing organic waste such as livestock excretion, food category waste, sewage and wastewater treatment sludge to produce biogas e.g. methane.    ADVANTAGE - Since the inner container is placed inside the outer container, the establishment area of the digestion tank can be remarkably reduced. Since the agitating blade is added to the inner side of the inner container, the mixing efficiency of the anaerobic digester can be improved. The acid fermenter and digeater are stirred at the same time, so that the deactivation of the methanogen by the acid-forming bacteria is prevented in the optimum pH range. Hence, the high-quality methane gas collection and high organic material elimination efficiency can be secured and the purity of the biogas can be improved. The organic elimination efficiency of the digeater can be enhanced with the dwell time of the methane fermentation from the narrow place. The organic waste can be steadily dispatched to the rotating inner container. Since a support roller is provided to support the circumference bottom plane of the inner container, the amount of administration of the organic waste can be increased and the processing capability of the anaerobic digester can be improved. Since a heat exchanger is installed in the inner wall of the outer container, the temperature of the digeater can be uniformly maintained and the activation of the methanogen can be maintained.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional view of the horizontal anaerobic digester. (Drawing includes non-English language text)    Outer container (10)    Inner container (30)    Outlet (35)    Supporting shafts (50a,50b)    Agitating blade (90)