PURPOSE: To acclimatize soil-borne facultative anaerobic bacteria and aerobic bacterial to produce a metabolite containing phenol or/and a compound containing exposed phenol group and cultivate the bacteria.

CONSTITUTION: Raw water containing organic materials, e.g. human and animal raw sewage waste water, fish processing waste water, agricultural processing waste water, etc., is fed to an adjusting step and cultivating step one after another to cultivate facultative anaerobic bacteria and aerobic bacteria in the soil bacterial group. In the adjusting step, the soil bacterial group living in the raw water staying on the above-mentioned step is acclimatized to produce a metabolite containing phenol or/and a compound containing exposed phenol group. In the cultivating step, the amount of the above-mentioned metabolite produced by the activity of the acclimatized soil bacterial group is increased and bacteria other than the soil bacterial group are inactivated by antibacterial action of the above-mentioned metabolite at the same time.