TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Nucleic Acid: (I) is composed of:(a) 17-30 nucleotides, and hybridizes with the DNA of dechlorination bacteria preferentially at the chlorinated ethylene-decomposing gene, where the nucleic acid has any of 46 base sequences (S1) given in the specification, such as taaccttctaaaacatcctg, gcaccacccataataatcta and atgtttgaaattccgcattg, any base sequences having homologies of 90% or higher with respect to the base arrangements of the base sequences mentioned above or any base sequences complementary to the afore- and above-mentioned base sequences,(b) 10-50 nucleotides, and hybridizes with the DNA of dechlorination bacteria preferentially at the chlorinated ethylene-decomposing gene, where the nucleic acid includes a base sequence of at least 10 succeeding nucleotides, which corresponds to a base sequence found (S1) or a base sequence complementary to the afore-mentioned base sequence, and(c) 10-50 nucleotides, and hybridizes with the DNA of dechlorination bacteria preferentially in the region from 1-500 base from the chlorinated ethylene-decomposing gene tceA upstream of it, where the nucleic acid has any of 7 base sequences (S2) given in the specification, such as atgtttgaaattccgcattg and tctccaactaactcgctttgtgactattat, any of the base sequences each having a homology of at least 90% with respect to the base arrangement of corresponding one of the above-mentioned base sequences or any of base sequences complementary to the afore and above-mentioned base sequences.(I) is preferably selected from the nucleic acid which is composed of 10-50 nucleotides and hybridizes with the DNA of dechlorination bacteria preferentially in a region adjoining the chlorinated ethylene-decomposing gene tceA upstream of the DNA, which region consists of the base sequence of a sequence (S3) comprising 427 amino acids given in the specification, where the hybridization is effected preferentially in such a range that includes at least one base disposed at the position of (S3) given in the specification, the nucleic acid which has a homology of at least 90% with respect to the base arrangement of the above nucleic acid, and the nucleic acid which has a base sequence complementary to the above nucleic acid, or the nucleic acids each of which is composed of 10-50 nucleotides and hybridizes with the DNA of dechlorination bacteria preferentially in the region from 1-500 base upstream of the chlorinated ethylene-decomposing gene tceA, where the nucleic acids includes each a base sequence of at least 10 succeeding nucleotides, which base sequence is found in S2, but not found in the base sequences of 7 sequences (S4) given in the specification, such as atatttgtagtttcgcattg and tctccaactaactcgttttgtgactattat, and nucleic acids having base sequences complementary to the afore-mentioned nucleic acids. EXAMPLE - DNAs were extracted from eight underground waters from eight polluted areas where microbiological decomposition of chlorinated ethylene has been confirmed, thus polymerase chain reaction (PCR) synthesis of DNA fragments including entire range of the chlorinated ethylene-decomposing gene was performed. The primer pair (5'-TAACCTTCTAAAACATCCTG-3' and 5'-GCACCACCCATAATAATCTA-3') were designed from the base sequence data of AF228507 registered in the GenBank in USA, where an adjoining region of the chlorinated ethylene-decomposing gene was utilized. The base sequence data of AF228507 include the chlorinated ethylene-decomposing gene tceA registered. By the PCR synthesis, clones which had high homologies with respect to the base arrangement of the chlorinated ethylene-decomposing gene tceA were obtained from four underground waters, of which base sequences were determined. The base sequence of the chlorinated ethylene-decomposing gene was the same for all the four underground water samples and had a homology of 96% with respect to the reported base arrangement of the gene tceA. The entire base sequence of the chlorinated ethylene-decomposing gene was determined. The base sequence of the chlorinated ethylene-decomposing gene was 1665 nucleotides fully defined in the specification.