NOVELTY - A microbial population obtained by administering aromatic chlorine compound into swamp reduction zone soil containing a population of soil microorganisms, where the soil is not contaminated with aromatic chlorine compound, culturing the soil microorganisms under anaerobic conditions, where the microbial population exhibits anaerobic degradation activity with respect to aromatic chlorine compound including polycyclic aromatic chlorine compound, maintains degradation activity even after five subculture or more or more than 10 months.    USE - The microbial population is useful for anaerobic degradation of aromatic chlorine compound including polycyclic aromatic chlorine, for purification of contaminated soil and for preventing diffusion of soil polluting substances (claimed).    ADVANTAGE - The microbial population has effective degradation activity with respect to aromatic chlorine compound, preferably polychlorinated biphenyl (PCB) even after 5 subculture or more and maintains degradation activity for more than 10 months.    DETAILED DESCRIPTION - A microbial population obtained by administering aromatic chlorine compound into swamp reduction zone soil containing a population of soil microorganisms, where the soil is not contaminated with aromatic chlorine compound, culturing the soil microorganisms under anaerobic conditions and where the microbial population exhibits anaerobic degradation activity with respect to aromatic chlorine compound including polycyclic aromatic chlorine compound, maintains degradation activity more than the initial stage degradation activity even after five subculture or more or more than 10 months. The microbial population comprises anaerobic microorganisms of Clostridium and microorganisms of genus Proteobacteria subclass delta and ~I and microorganisms comprising only 6,7 menaquinone in the microbial population structure analysis by quinone profile method, where the decomposition reaction is dechlorination reaction in meta and para position of aromatic ring, and dechlorination activity with respect to anyone of 2-chlorinated compound to 6-chlorinated compound detected in soil.    INDEPENDENT CLAIMS are included for:    (1) an artificial culture medium for inoculating the microbial population exhibiting anaerobic degradation activity with respect to aromatic chlorine compound, comprising arbitrary microorganisms culture medium and an artificial porous material having several pore openings, where the culture medium maintains the anaerobic degradation activity of the inoculated microbial population for more than 10 months and has increased (two times or more) degradation activity;    (2) a microbial population composition obtained by inoculating the microbial population exhibiting anaerobic degradation activity with respect to aromatic chlorine compound including polycyclic aromatic chlorine compound into the artificial culture medium and culturing under anaerobic conditions;    (3) increasing or maintaining the activity of microbial population which involves inoculating the microbial population exhibiting anaerobic degradation activity with respect to aromatic chlorine compound including polycyclic aromatic chlorine compound into the artificial culture medium and culturing under anaerobic conditions;    (4) purification of contaminated soil which involves mixing the microbial population or the microbial population composition with the soil contaminated with aromatic chlorine compound; and    (5) a method for preventing diffusion of soil polluting substance which involves preparing a barrier layer containing a microbial population or a microbial population composition in the soil or around the soil contaminated with the aromatic chlorine compound or in the bottom layer of the soil in horizontal direction and preventing the diffusion of aromatic chlorine compound to the outer region of barrier layer.    DESCRIPTION OF DRAWING(S) - The figure is a graph representing PCV degradation activity of the subcultured microbial population. (Drawing includes non-English language text)