Experimental study of anaerobic digestion process combined with and without the submerged separation membrane was investigated by using laboratory-scale reactor at the hydraulic retention time of 0.5 day and 1.0 day. The removal efficiencies of carbohydrate at the digester without and with membrane were 84.4 to 86.8 % and 99.6 to 99.7 %, respectively, and the methane gas recovery efficiencies were 0.4 to 1.2 % and 12.3 to 28.7 %. According to the measurement by the most probable numbers method, the numbers of various groups of bacteria in the digesters with membrane were enumerated in the following ranges ; acidogens 1.7×10^9 to 5.0 × 10^9 MPN/㎖, homoacetogens : 5.0×10^7 to 2.4×10^8 MPN/㎖, H₂-utilizing methanogens : 1.3×10^7 to 9.2×l0^8 MPN/㎖, and acetate-utilizing methanogens : 2.3×10^6 to 2.0×10^8 MPN/㎖. The number of methanogens at the digester with membrane increased by approximately 10² to 10⁴ times in comparison with that of the digester without membrane.