This invention is related to a production method of a branched polymer which comprises polymerizing a macromonomer [I], said macromonomer [I] being a vinyl polymer obtainable by radical polymerization and terminally having one polymerizable carbon-carbon double bond-containing group per molecule. Furthermore, by producing the macromonomer by living radical polymerization, in particular atom transfer radical polymerization, it becomes possible to produce the above polymers or gels having a well controlled side chain molecular weights.