TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Method: The method (M1) involves (a) monitoring ratio of the DNA and RNA concentration of the various genuine bacteria in connection with acid production and whole genuine bacteria group or ratio of the DNA and RNA concentration of various methane formation microorganisms and whole methane formation microorganism group, and estimation evaluation of stability of activity of methane fermentation type from the above change and fixing condition within the system of each microorganism species, (b) carrying out assay detection of DNA and RNA concentration for every group which are classified based on the substrate utilization of the genuine bacteria in connection with the acid production in methane fermentation microorganisms type specifically using the primer set containing the oligonucleotide comprising (i) nucleotide sequence having a fully defined 19 nucleotides (SEQ ID No. 7) sequence given in the specification, which targeted cellulose decomposition acid production bacteria Clostridium sp. JC3 strain (AB093546) and its close-relation stock Clostridium sp. clone 6-2-K(AB198871), clone 5-1-N(AB197849) or nucleotide sequence substantially homologous to SEQ ID No. 7 and nucleotide sequence having a fully defined 18 nucleotides (SEQ ID No. 8) sequence given in the specification or nucleotide sequence substantially homologous with SEQ ID No. 8, (ii) nucleotide sequence having a fully defined 23 nucleotides (SEQ ID No. 9) sequence given in the specification, which targeted cellulose decomposition acid production bacteria Clostridium sp. JC94 clone(AB231801) or nucleotide sequence substantially homologous with SEQ ID No. 9 and nucleotide sequence of SEQ ID No. 8 or base sequence substantially homologous to SEQ ID No. 8, (iii) nucleotide sequence having a fully defined 20 nucleotides (SEQ ID No. 10) sequence given in the specification, which targeted proteolysis acid production bacteria Coprothermobacter proteolyticus strain P1 (AB162803) or nucleotide sequence substantially homologous to SEQ ID No. 10 and nucleotide sequence having a fully defined 16 nucleotides (SEQ ID No. 11) sequence given in the specification or nucleotide sequence substantially homologous to SEQ ID No. 11, (iv) nucleotide sequence having a fully defined 22 nucleotide (SEQ ID No. 12) sequence given in the specification, which targeted acid production bacteria uncultured bacterium clone S1 (AB239188) in connection with decomposition of starch and glucose or nucleotide sequence substantially homologous with SEQ ID No. 12 and nucleotide sequence having a fully defined 20 nucleotides (SEQ ID No. 13) sequence given in the specification or nucleotide sequence substantially homologous with SEQ ID No. 13, and (v) nucleotide sequence having a fully defined 24 nucleotides (SEQ ID No. 14) sequence given in the specification, which targeted acid production bacteria uncultured bacterium clone S2 (AB239189) in connection with decomposition of starch and glucose or nucleotide sequence substantially homologous with SEQ ID No. 14 and nucleotide sequence having a fully defined 23 nucleotides (SEQ ID No. 15) sequence given in the specification or nucleotide sequence substantially homologous with SEQ ID No. 15, (c) carrying out assay detection of DNA and RNA concentration of genuine bacteria group comprehensively using PCR method using oligonucleotide comprising nucleotide sequence having a fully defined 17, 20 or 25 nucleotides (SEQ ID No. 1-3) sequence given in the specification or nucleotide sequence substantially homologous with SEQ ID No. 1-3, (d) monitoring ratio of DNA and RNA concentration of various genuine bacteria in connection with the acid production obtained at the step (b) and DNA and the RNA concentration of the whole genuine bacteria group which are obtained at the step (c), and (e) carrying out estimation evaluation of stability of activity of methane fermentation type and fixing condition in various genuine bacterial system from the above change, or (f) carrying out assay detection of DNA and the RNA concentration of methane formation microorganism in methane fermentation microorganisms type specifically on genus or family level using the primer set containing the oligonucleotide comprising (i) nucleotide sequence having a fully defined 21 nucleotides (SEQ ID No. 16) sequence given in the specification, which targeted the Methanosarcina genus microorganism group or nucleotide sequence substantially homologous with SEQ ID No. 16 and nucleotide sequence having a fully defined 17 nucleotides (SEQ ID No. 17) sequence given in the specification or nucleotide sequence substantially homologous with SEQ ID No. 17, (ii) nucleotide sequence having a fully defined 18 nucleotides (SEQ ID No. 18) sequence given in the specification, which targeted the Methanosaetaceae family microorganism group or nucleotide sequence substantially homologous with SEQ ID No. 18 and nucleotides sequence having a fully defined 23 nucleotides (SEQ ID No. 19) sequence given in the specification or nucleotide sequence substantially homologous to SEQ ID No. 19, (iii) nucleotide sequence having a fully defined 23 nucleotides (SEQ ID No. 20) sequence given in the specification, which targeted Methanoculleus genus of Methanomicrobiaceae family or nucleotide sequence substantially homologous with SEQ ID No. 20 and nucleotides sequence having a fully defined 23 nucleotides (SEQ ID No. 21) sequence given in the specification or nucleotide sequence substantially homologous to SEQ ID No. 21, (iv) nucleotide sequence having a fully defined 22 nucleotides (SEQ ID No. 22) sequence given in the specification, which targeted the Methanospirillaceae family microorganism group of Methanomicrobiales order or nucleotide sequence substantially homologous with SEQ ID No. 22 and nucleotides sequence having a fully defined 24 nucleotides (SEQ ID No. 23) sequence given in the specification or nucleotide sequence substantially homologous to SEQ ID No. 23, and (v) nucleotide sequence having a fully defined 22 nucleotides (SEQ ID No. 24) sequence given in the specification, which targeted Methanobacteriaceae family microorganisms group or nucleotide sequence substantially homologous to SEQ ID No. 24 and nucleotide sequence having a fully defined 22 nucleotides (SEQ ID No. 25) sequence given in the specification or nucleotide sequence substantially homologous to SEQ ID No. 25, (g) carrying out fixed assay detection of DNA and RNA concentration of methane formation microorganism group comprehensively using the PCR method using oligonucleotide comprising nucleotide sequence having a fully defined 17, 20 or 25 nucleotides (SEQ ID No. 4-6) sequence given in the specification or nucleotide sequence substantially homologous to SEQ ID No. 4-6, (h) monitoring the ratio of DNA and the RNA concentration of various methane formation microorganisms obtained at the step (f) and DNA and RNA concentration of the whole methane formation microorganism group obtained at the step (g) with time, and carrying out estimation evaluation of stability of activity of methane-fermentation system from the above change and the fixing condition in systems of various methane formation microorganisms.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Method: The volatile organic acid is acetic acid, and ratio of acetic acid generation rate and methane generation rate is maintained at 2.5 or less.