TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Method: The detection process involves evaluating methane-formation ability by investigating the dominant situation of the predetermined methane-formation microbe group in the methane-fermentation microorganisms system.The method involves performing PCR, and estimating the methane-formation efficiency by monitoring with time the microbe concentration and/or activity of a predetermined methane-formation microbe group.The method further involves evaluating cellulose decomposability by detecting 16S rRNA gene having at least 90% homology with respect to 16S rRNA having a fully defined 1623, 1617 or 1610 base pair (SEQ ID No. 15, 16 or 17) sequence given in the specification of a Clostridium sp. JC3 strain, and/or 16S rRNA (SEQ ID No. 14) of the close-relation stock Clostridium sp. JC94 strain from the nucleic acid sample sampled from the methane-fermentation microorganism group, and investigating the dominant situation of the cellulose decomposition acid production microbe group in the methane-fermentation microorganism group.The method involves evaluating cellulose decomposability by measuring the microbe concentration and/or activity of a predetermined cellulose degradation acid production microbe group, or estimating cellulose degradation efficiency by monitoring temporally the microbe concentration and/or activity of a predetermined cellulose degradation acid production microbe group.The method involves performing PCR assay using nucleic acid sample sampled from the methane-fermentation microorganism group and the primer set for detecting methane producing microbes at genus or family level, monitoring the microbe concentration and/or activity of a determined methane-formation microbe group with time, performing PCR assay using nucleic acid sample sampled from the methane-fermentation microorganism group and the primer set for detecting cellulose degradation acid producing microbes at species level, monitoring the microbe concentration and/or activity of a predetermined cellulose decomposition acid production microbe group with time, and estimating the stability of the methane-formation microorganism group by evaluating the balance between the cellulose decomposition efficiency and methane-formation efficiency in the methane-fermentation microorganism group based on the two monitoring result. EXAMPLE - No relevant example given.