A nutrient solution is reduced with a reducing agent, and introduced into a contaminated object. Alternatively, groundwater reduced with a reducing agent is introduced into the contaminated object. Groundwater is contacted with the reducing agent while being circulated between an aquifer and a housing. A nutrient source for heterotrophic anaerobic microorganism may also be introduced into the contaminated object. The contaminated object contaminated with halogenated organic compounds can be purified efficiently and easily.