NOVELTY - An apparatus for producing bio-gas comprises: an anaerobic digestion reactor (30) to which part of raw material is fed; a first solid-liquid separator (10) to which other part of raw material is fed, where other part contains inhibitory materials; a hydrolysis reactor (20) that executes hydrolysis of solid-phase material obtained from first solid-liquid separator and then feeds treated material into anaerobic digestion reactor; and a sludge concentrator (50) that concentrates liquid-phase material obtained from first solid-liquid separator as well as a liquid waste of reactor.    USE - As apparatus for producing bio-gas (claimed).    ADVANTAGE - The apparatus is capable of improving quality of a raw material fed into it, thus maintains desired activity of microorganisms used for producing bio-gas. Additionally, by returning a concentrated sludge containing a large amount of anaerobic microorganisms to a digestion reactor, production of bio-gas by the anaerobic microorganisms is maximized while equipment costs are decreased, thus ensures favorable efficiency.    DETAILED DESCRIPTION - An apparatus for producing bio-gas, comprises: an anaerobic digestion reactor (30) to which a part of a raw material is fed, where the part does not contain inhibitory materials; a first solid-liquid separator (10) to which the other part of the raw material is fed, where the other part contains the inhibitory materials, and which separates this part into a solid-phase material free from the inhibitory materials and a liquid-phase material containing the inhibitory materials; a hydrolysis reactor (20) that executes hydrolysis of the solid-phase material obtained from the first solid-liquid separator and then feeds the treated material into the anaerobic digestion reactor; and a sludge concentrator (50) that concentrates the liquid-phase material obtained from the first solid-liquid separator as well as a liquid waste of the anaerobic digestion reactor. An INDEPENDENT CLAIM is included for producing bio-gas, involving: a) separating a part of a raw material, which contains inhibitory materials, into a solid-phase material free from the inhibitory materials and a liquid-phase material containing the inhibitory materials; b) hydrolyzing the solid-phase material generated in step (a); c) conducting anaerobic digestion of the other part of the raw material, which does not contain the inhibitory materials, as well as the hydrolyzed material formed in step (b), so as to produce bio-gas; and d) subjecting the liquid-phase material separated in step (a) and a liquid waste formed by anaerobic digestion in step (c) to membrane separation, in order to concentrate the same.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic diagram of the apparatus for production of bio-gas.    First solid-liquid separator (10)    Hydrolysis reactor (20)    Anaerobic digestion reactor (30)    Second solid-liquid separator (40)    Sludge concentrator (50)