To obtain a treatment agent having a high treatment capacity of nitric acid phase and nitrous acid phase nitrogen-containing compounds at low costs, used at a normal atmospheric pressure, hardly exerting influence on the environment, and excellent in stability.

Materials containing a reducing agent such as sodium hydrogen sulfate and/or a metal such as iron, a metal oxide such as iron oxide, titanium oxide, are used as the treatment agent. The treatment agent has an excellent capacity to treat the hazardous nitric acid phase and nitrous acid phase nitrogen-containing compounds at the normal atmospheric pressure, hardly causes a secondary deterioration in the environment and soil, and uses low-cost materials such as the metal oxide. Thereby, the treatment agent is useful as the treatment material for drainage or soil containing the nitric acid phase and nitrous acid phase nitrogen-containing compounds.