NOVELTY - Purifying (M1) contaminated soil, involves adding water containing nitrate nitrogen and/or nitrite nitrogen to contaminated soil, and further adding denitrifying bacteria such as Thiobacillus denitrificans having a fully defined 421 nucleotide (SEQ ID No. 1) sequence given in the specification, in 16S rRNA gene, such that the contaminant in the contaminated soil is degraded.    USE - (M1) is useful for purifying contaminated soil, where the contaminant comprises one or more types of halogen-containing hydrocarbons, oil component resulting from crude oil and from petroleum product purified from crude oil, oil component resulting from coal product, animal and vegetable oils, or monocyclic aromatic compound hydrocarbon having as main constituent, benzene, phenanthrene or fluorene, or polycyclic aromatic hydrocarbon having as main constituent, pyrene, tetrachloroethylene or trichloroethylene, as main constituent. (M2) is useful for purifying contaminated ground water (claimed).    ADVANTAGE - (M1) enables efficient, rapid and stable purification of contaminated soil. (M1) enables highly efficient removal of contaminant from soil or underground water.    DETAILED DESCRIPTION - Purifying (M1) contaminated soil, involves:    (a) adding water containing nitrate nitrogen and/or nitrite nitrogen to contaminated soil; and    (b) further adding denitrifying bacteria such as Thiobacillus denitrificans having a fully defined 421 nucleotide (SEQ ID No. 1) sequence given in the specification, in 16S rRNA gene; Sterolibacterium denitrificans having one of two fully defined 421 nucleotide sequence (SEQ ID No. 2 or 3) given in the specification, in 16S rRNA gene; Stenotrophomonas nitritireducens having one of two fully defined 421 nucleotide sequence (SEQ ID No. 4 or 5) given in the specification, in 16S rRNA gene; or Diaphorobacter nitroreducens having one of two fully defined 419 nucleotide sequence (SEQ ID No. 6 or 7) given in the specification, in 16S rRNA gene, where the denitrifying bacteria existing in contaminated soil is utilized and the contaminant in the contaminated soil is degraded.    INDEPENDENT CLAIMS are included for:    (1) purifying (M2) contaminated ground water comprising nitrite nitrogen or nitrate nitrogen as contaminant, involves adding one or more types of denitrifying bacteria of (M1) to the contaminated ground water, such that the contaminant in the contaminated ground water is degraded; and    (2) monitoring procedures of the denitrifying bacteria, involves detecting the denitrifying bacteria of (M1) based on the base sequence of SEQ ID No. 1-7, and obtaining over-time change of increase and decrease of the denitrifying bacteria in the contaminated soil, or its spatial distribution amount.