To provide a method for producing an aqueous cadaverine salt solution whereby polymeric impurities can be removed from the solution.

An aqueous cadaverine salt solution produced from lysine by the use of a lysine decarboxylase, a recombinant microorganism with an improved lysine decarboxylation activity, or cells or processed cells which produce a lysine decarboxylase is filtered through an ultrafilter membrane to remove polymeric impurities, (e.g., proteins) of a molecular weight of 12,000 or higher from the solution. A polyamide resin made from the treated cadaverine contains little gel and is therefore desirable for films, injection moldings, fibers, monofilaments, or the like.