A method of preventing polymer scale deposition in a polymerization vessel during polymerization of a monomer having an ethylenically double bond, wherein said polymerization is carried out in a polymerization of which the inner wall has been previously coated with a coating solution comprising:(A) an aromatic amine-based condensate,(B) at least one member selected from the group consisting of alkali metal salts and ammonium salts of a sulfonation product prepared by sulfonation of an aromatic amine-based condensate, and(C) at least one member selected from the group consisting of metal compounds, inorganic colloids, dyes and pigments; a scale preventive agent comprising the components (A), (B) and (C); and a polymerization vessel having a coating comprising the scale preventive agent. According the method, polymer scale deposition on the inner wall, etc. of the polymerization vessel can be effectively prevented.