The present invention provides a chlorotrifluoroethylene copolymer endowed with stress cracking resistance, chemical resistance and thermal stability. The present invention is a chlorotrifluoroethylene copolymer constituted of chlorotrifluoroethylene unit, tetrafluoroethylene unit and a monomer [A] unit derived from a monomer [A] copolymerizable with chlorotrifluoroethylene and tetrafluoroethylene, wherein the sum of chlorotrifluoroethylene unit and the tetrafluoroethylene unit amounts to 90 to 99.9 mole percent and the monomer [A] unit amounts to 0.1 to 10 mole percent.