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 vessel of which the inner wall has been previously first coated with
(A) a coating solution comprising a condensate of an aromatic compound and at least one member selected from the group consisting of metal compounds and inorganic col­loids, and the coating thus formed has been then coated with (B) a coating solution comprising at least one member selected from the group consisting of anionic polymeric compounds, amphoteric polymeric compounds and hydroxyl group-containing polymeric compounds; and a polymerization vessel having a coating thus formed. According this method, polymer scale deposition on the inner wall, etc. of the polymerization vessel can be effectively prevented.