To provide a persistent prevention method of polymn. scale in a polymn. with water as a medium without using any org. solvent at all.

A polymn. scale preventing agent is emulsified in a water medium by dissolving it in an aq. alkaline solution, and then neutralized with an acid compd., and this coating soln. is coated on the inner wall of a polymn. reactor, and dried before polymn. This polymn. scale preventing agent is pref. at least one selected from the group consisting of pyrogallol/benzaldehyde condensate, pyrogallol/formalin condensate, and pyrogallol-formalin-resorcinol condensate.