There is provided a polymer scale deposition preventive agent, comprising an alkaline solution of a condensation product having a molecular weight of 500 to 1,000,000 obtained by condensing a dimer compound of 2,3-dihydroxynaphthalene with formaldehyde. By polymerizing vinyl chloride or a monomer mixture containing vinyl chloride in a polymerization vessel wherein a coating film made of said polymer scale deposition preventive agent has been formed on the inner wall surface and the like, polymer scale is effectively prevented from depositing not only on an agitation apparatus but also on areas near the interface between a gas phase and a liquid phase, which prevention of the polymer scale deposition is conventionally difficult. Thus, a vinyl chloride polymer can be obtained which contains colored foreign matters, fish eyes, etc. in small amounts and is small in initial discoloration and is good in quality.