The purpose of the present invention is to provide a polymer having a low halogen atom content remaining in the polymer, a simple production method thereof, an active energy ray-curable composition that can be rapidly cured by an irradiation of a small amount of light, and a cured product thereof. These purpose can be achieved by an active energy ray-curable composition, including a polyisobutylene polymer (A) represented by the following general formula (1) (wherein R1 represents a monovalent or polyvalent aromatic hydrocarbon group, or a monovalent or a polyvalent aliphatic hydrocarbon group; A represents a polyisobutylene polymer; R2 represents a divalent saturated hydrocarbon group having 2-6 carbon atoms, which contains no hetero atoms; R3 and R4 each represent hydrogen, a monovalent hydrocarbon group having 1-20 carbon atoms, or an alkoxy group having 1-20 carbon atoms; R5 represents hydrogen or a methyl group; and n denotes a natural number), and an active energy ray polymerization initiator (B).