NOVELTY - An anaerobic digestion processing apparatus has an anaerobic tank, a modifier (6), a modified sludge return unit (7), a solid-liquid separator (3), and a thickened sludge return unit (4). The anaerobic digestion tank has two or more tanks, e.g. a thermophilic digestion tank (1) with a processing temperature of 45-95degreesC, and a mesophilic digestion tank (2) with a processing temperature of 25-40degreesC into which effluent of the thermophilic digestion tank is introduced. One or more portion(s) of digested sludge from the mesophilic digestion tank is/are introduced into the separator.    USE - Used for anaerobic-digestion processing of organic waste liquid (claimed), such as organic sludge, human waste, and foodstuffs industry waste water.    ADVANTAGE - The anaerobic digestion processing apparatus enables efficient processing of organic waste liquid even under high organic matter loading, and reduces the amount of organic substances without the need for increasing a heating energy or the amount of a flocculant to be used. The apparatus enables methane gas to be recovered in an increased amount.    DETAILED DESCRIPTION - The anaerobic digestion processing apparatus has an anaerobic tank which carries out anaerobic digestion of organic waste liquid (AA), a modifier (6) for modifying a portion of digested sludge from the anaerobic digestion tank, a modified sludge return unit (7) to return the modified sludge from the modifier to an aerobic digestion tank, a solid-liquid separator (3) to concentrate the digested sludge from the anaerobic digestion tank, and a thickened sludge return unit (4) to return the thickened sludge from the solid-liquid separator to the anaerobic digestion tank. The anaerobic digestion tank has two or more tanks of a thermophilic digestion tank (1) with a processing temperature of 45-95degreesC, and a mesophilic digestion tank (2) with a processing temperature of 25-40degreesC into which the effluent of the thermophilic digestion tank is introduced. One or more portion(s) of the digested sludge from the mesophilic digestion tank is/are introduced into the solid-liquid separator.    DESCRIPTION OF DRAWING(S) - The figure shows the systematic diagram of the anaerobic digestion processing apparatus. (Drawing includes non-English language text).    Thermophilic-digestion tank (1)    Mesophilic-digestion tank (2)    Solid-liquid separator (3)    Thickened-sludge return unit (4)    Modifier (6)    Modified-sludge return unit (7)    Organic waste liquid (AA)