NOVELTY - A primer set (I) comprising a set of oligonucleotides, is new.    USE - (I) is useful for detecting methane fermenting microorganisms, which involves using nucleic acid sample from methane fermenting microorganisms, as a target, and at least one pair of (I), performing PCR, and detecting the amplified product obtained by PCR. The method involves: (a) utilizing nucleic acid sample of methane fermenting microorganisms as a target and (I), and performing first PCR; and (b) utilizing the amplified product obtained by first PCR, as a target, and performing second PCR, using (I). The method further involves subjecting the amplified product obtained by PCR, for a denaturant concentration gradient gel electrophoresis, after performing PCR, utilizing (I). The methane fermenting microorganisms with respect to a detection target, exist in an environmental sample. The detection method is useful for evaluating environmental sample, which involves using the distribution of the migration band, obtained by electrophoresis, by the detection method, as a parameter data of the presence of culture of methane fermenting microorganisms, in the environmental sample. The environmental sample is methane-fermentation sludge (claimed), including organic waste and wastewater. (I) is useful for detecting bacteria of Methanobacterium genus, in an environmental sample.    ADVANTAGE - (I) enables rapid, simple and effective detection of methane fermenting microorganisms, in environmental sample, comprehensively or specifically.    DETAILED DESCRIPTION - A primer set (I) comprising a set of oligonucleotides, is chosen from:    (a) oligonucleotide having sequence such as aacgcgtgagcaacctgcc (SEQ ID No. 1) or a base sequence substantially homologous to SEQ ID No. 1, and oligonucleotide having a sequence such as ctcatggtgtgacgggcggtgtg (SEQ ID No. 2) or a base sequence substantially homologous to SEQ ID No. 2, (b) oligonucleotide having a sequence such as aacgcgtgagcaacctgcc (SEQ ID No. 3) or a base sequence substantially homologous to SEQ ID No. 3, and oligonucleotide having a sequence such as gcgcgcggcgcgccccgcgcccgtcccgccgggcccgcggctcccgtaggagtctgg (SEQ ID No. 4) or a base sequence substantially homologous to SEQ ID No. 4; and    (c) an oligonucleotide having a sequence such as cttcaccgaaacacccttcgg (SEQ ID No. 5) or a base sequence substantially homologous to SEQ ID No. 5, and oligonucleotide having a sequence such as ataagggttccgctggtaactaag (SEQ ID No. 6) or a base sequence substantially homologous to SEQ ID No. 6.