To provide a treatment method by which variation in chloride ion concentration in pumped-up ground water is measured and thereby anaerobic microorganisms are activated to decompose a volatile organic chlorine compound to harmless low molecular compounds when the anaerobic microorganism having a spore, especially Clostridium species KD13, decomposes the volatile organic chlorine compound to the low molecular compounds.

An injection well 4 and a pumping-up well 5 reaching a lower part of a water permeable layer 3 at a soil lower part are installed in or near a contamination region 2. The anaerobic microorganisms and a nutrient for activating them are injected into the injection well 4, the ground water pumped up from the pumping-up well 5 is stored in a storing tank 9, the ground water in the storing tank 9 is made to flow into an aeration tank 10. After the aeration tank 10 is made in an aerobic state and sporulation of the anaerobic microorganism is executed, the anaerobic microorganisms are germinated in a microorganism cultivation tank 8 and then germinated anaerobic microorganisms are injected again in the injection well 4.