NOVELTY - Primer set for (i) detecting methane producing microbes at genus or family level, comprises oligonucleotide sets for detecting microorganisms of Methanosarcina, Methanoculleus and Methanogenium genus, microorganisms of Methanosaetaceae, Methanospirillaceae and Methanobacteriaceae family, and (ii) detecting cellulose degradation acid producing microbes at species level, comprises oligonucleotide set for detecting Clostridium sp. JC3 strain, and JC94 strain.    USE - For detecting methane producing microbes at genus or family level, and detecting cellulose degradation acid producing microbes at species level, and evaluating microorganisms processing activity such as methane-formation ability and cellulose decomposability in methane fermentation microorganism group (claimed) contained in organic waste and wastewater.    ADVANTAGE - The primer set enables selective detection of methane-formation microbes or acid production microbes in the methane fermentation microorganism group, at genus, family or species level, and enables to evaluate microorganism processing activity even in the presence of fluctuation in the number of microbes.    DETAILED DESCRIPTION - Primer set for:    (i) detecting methane producing microbes at genus or family level, comprises any one set of oligonucleotides chosen from:    (a) oligonucleotide set for detecting microorganisms of Methanosarcina genus, comprising oligonucleotide having base sequence of a fully defined 21 base pair (SEQ ID No. 1) sequence given in the specification or base sequence substantially homologous to SEQ ID No. 1, and oligonucleotide having base sequence of a fully defined 17 base pair (SEQ ID No. 2) sequence given in the specification or base sequence substantially homologous to SEQ ID No. 2;    (b) oligonucleotide set for detecting microorganisms of Methanosaetaceae family, comprising oligonucleotide having base sequence of a fully defined 18 base pair (SEQ ID No. 3) sequence given in the specification or base sequence substantially homologous to SEQ ID No. 3, and oligonucleotide having base sequence of a fully defined 23 base pair (SEQ ID No. 4) sequence given in the specification or base sequence substantially homologous to SEQ ID No. 4;    (c) oligonucleotide set for detecting microorganisms belonging to Methanoculleus and Methanogenium genus of Methanomicrobiaceae family, comprising oligonucleotide having base sequence of a fully defined 23 base pair (SEQ ID No. 5) sequence given in the specification or base sequence substantially homologous to SEQ ID No. 5, and oligonucleotide having base sequence of a fully defined 23 base pair (SEQ ID No. 6) sequence given in the specification or base sequence substantially homologous to SEQ ID No. 6;    (d) oligonucleotide set for detecting microorganisms of Methanospirillaceae family of Methanomicrobiales order, comprising oligonucleotide having base sequence of a fully defined 22 base pair (SEQ ID No. 7) sequence given in the specification or base sequence substantially homologous to SEQ ID No. 7, and oligonucleotide having base sequence of a fully defined 24 base pair (SEQ ID No. 8) sequence given in the specification or base sequence substantially homologous to SEQ ID No. 8; and    (e) oligonucleotide set for detecting microorganisms belonging to Methanobacteriaceae family, comprising oligonucleotide having base sequence of a fully defined 22 base pair (SEQ ID No. 9) sequence given in the specification or base sequence substantially homologous to SEQ ID No. 9, and oligonucleotide having base sequence of a fully defined 22 base pair (SEQ ID No. 10) sequence given in the specification or base sequence substantially homologous to SEQ ID No. 10; or    (ii) detecting cellulose degradation acid producing microbes at species level, comprises any one set of oligonucleotides chosen from:    (a) oligonucleotide set for detecting Clostridium sp. JC3 strain, comprising oligonucleotide having base sequence of a fully defined 19 base pair (SEQ ID No. 11) sequence given in the specification or base sequence substantially homologous to SEQ ID No. 11, and oligonucleotide having base sequence of a fully defined 18 base pair (SEQ ID No. 12) sequence given in the specification or base sequence substantially homologous to SEQ ID No. 12; and    (b) oligonucleotide set for detecting Clostridium sp. JC94 strain, comprising oligonucleotide having base sequence of a fully defined 23 base pair (SEQ ID No. 13) sequence given in the specification or base sequence substantially homologous to SEQ ID No. 13, and oligonucleotide having base sequence of a fully defined 1628 base pair (SEQ ID No. 14) sequence given in the specification or base sequence substantially homologous to SEQ ID No. 14.    An INDEPENDENT CLAIM is included for method for evaluating microorganisms processing activity in methane fermentation microorganism group, involves detecting microbe group associated with methane formation in the methane-fermentation microorganism group by performing PCR using nucleic acid sample obtained from the methane-fermentation microorganism group, and the primer set mentioned above.