A polymer scale preventive agent for use in polymerization of a monomer having an ethylenically unsaturated double bond, comprising a condensation product obtained by condensing

(A) an aliphatic diamine compound, and
(B) a quinone compound.

in an aqueous medium. Where the monomer is polymerized in a polymerization vessel having, on its inner wall surface, a coating comprising the preventive agent, deposition of polymer scale can be effectively prevented not only on the liquid phase part in the polymerization vessel but also near the interface between the gas phase part and the liquid phase part, so that the thus obtained polymer can provide a molded item having few fish eyes and low in initial coloration, when the polymer is molded into a sheet or the like.