PURPOSE: To stabilize a bromous acid group, by reacting bromous acid or its salt with a quaternary ammonium group-containing polymer to form a bromous acid quaternary ammonium group-containing polymer.

CONSTITUTION: A quaternary ammonium group-containing polymer such as a strong basic anion exchange resin, etc. is immersed in a solution of bromous acid or its salt, and, if necessary, stirring is carried out, so that ion exchange reaction occurs quickly, to form a bromous acid quaternary ammonium group-containing polymer. The state is also observed by the color change of the polymer. The prepared polymer is separated from the reaction solvent by a method, e.g., ordinary decantation, filtration, etc. The polymer is extremely stable and not decomposed, through being allowed to stand for many hours in air. It is usable as an oxidizing agent in various chemical reactions.