A method for producing a vinyl polymer including a polymerization step of using a composition including a microcapsule and polymerizing a vinyl monomer by radical reaction to produce a vinyl polymer, in which the microcapsule has a core/shell structure, and the shell includes a water-soluble polymer and the core includes an organic peroxide. The water-soluble polymer is preferably at least one water-soluble polymer selected from the group consisting of polyvinyl alcohol, cellulose derivatives, gelatin, poly(meth)acrylic acid derivatives, polyvinylpyrrolidone, and polyethylene oxide.