To provide a method for preparing a nitrite/nitrate-nitrogen lowering agent wherein an agent constituted of a water soluble cationic polymer compound and a polymeric material is provided with capability of more efficiently and more simply lowering nitrite/nitrate-nitrogen concentration in water in a shorter time relative to a prior art method for lowering nitrite/nitrate-nitrogen concentration, and a method for lowering nitrite/nitrate-nitrogen concentration in water using the nitrite/nitrate-nitrogen lowering agent prepared by the above preparation method.

The method for preparing a nitrite/nitrate-nitrogen lowering agent is characterized by causing a water soluble cationic polymer compound to adhere to wool in the presence of an alkali and a reducing agent. The method for lowering nitrite/nitrate-nitrogen concentration in water comprises adding the nitrite/nitrate-nitrogen lowering agent into water comprising nitrite/nitrate-nitrogen, causing the lowering agent to adsorb nitrite/nitrate-nitrogen by stirring/mixing water and the lowering agent, subsequently adding an anionic polymeric flocculant or inorganic flocculant thereinto, stirring/mixing the resulting mixture, and thereafter carrying out solid/liquid separation by causing the lowering agent having adsorbed nitrite/nitrate-nitrogen to settle.