The present invention provides a vinyl polymer with an epoxy group terminally introduced therein and an epoxy resin composition comprising (A) an epoxy resin and (B) a vinyl polymer which has a main chain produced by living radical polymerization and has a reactive functional group at a main chain terminus. A flexible epoxy resin composition can be obtained using these compounds. The invention further provides an epoxy resin composition having an epoxy group at a main chain terminus thereof.