A copolymer comprising a structural unit (I) derived from a phosphonate monomer (1) such as diethyl methacryloyloxymethyl phosphonate, diethyl 2-methacryloyloxyethyl phosphonate and the like, and a structural unit (II) derived from a radical polymerizable monomer (2) such as methacrylic acid, acrylic acid, 8-tricyclo[5.2.1.02,6]decanyl methacrylate, t-butyl methacrylate and the like, wherein the content of phosphorus atoms from the phosphonate monomer (1) is 2.0 to 6.0% by mass relative to the mass of the copolymer. A formed article comprising the copolymer.