To provide a method for preventing polymer adhesion to a vapor phase part in a polymerization reactor even in polymerization of a vinyl monomer containing an acrylate.

In a polymerization method of a vinyl monomer containing not less than 2 mass % of an acrylate using a polymerization reactor, the polymerization method of the vinyl monomer comprises existence of at least one 3-5C mercaptan in a vapor phase part in a polymerization reactor wherein preferably concentration of the 3-5C mercaptan in the vapor phase part is within a range of 10-1,000 ppm and further the mercaptan with not less than 6C can also exist in the vapor phase part.