A nucleic acid composed of 17 30 nucleotides, which hybridizes with the DNA of dechlorination bacteria preferentially at the chlorinated ethylene-decomposing gene, or a nucleic acid composed of 10 50 nucleotides, which hybridizes with the DNA of dechlorination bacteria preferentially in a region containing the base sequence(s) characteristic to high performance dechlorination bacteria present upstream from the chlorinated ethylene-decomposing gene tceA, to be used for detecting selectively dechlorination bacteria, in particular high performance dechlorination bacteria, by performing a PCR using the nucleic acid mentioned above as the primer and the nucleic acid(s) in the sample bacteria as the template and detecting the thereby synthesized DNA fragments, in order to use the thus identified dechlorination bacteria for an efficient treatment of earth or underground waters polluted by chlorinated ethylene and/or chlorinated ethane.