A copolymer comprising a structural unit (I) derived from a phosphonate monomer (1) such as diethyl methacryloyloxymethyl phosphonate and diethyl 2-methacryloyloxyethyl phosphonate, 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, and t-butyl methacrylate, and a structural unit (III) derived from methyl methacrylate, the content of phosphorus atoms derived from the phosphonate monomer (1) being not less than 1.0% by mass and less than 2.0% by mass relative to the mass of the copolymer, and a formed article comprising the copolymer.