The present invention is to provide a fluorine-containing copolymer consisting essentially of monomer units (A) of chlorotrifluoroethylene and monomer units (B), and having a ratio of the number of moles of the monomer unit (A) to the total number of moles of the monomer unit (A) and the monomer unit (B) of from 3 to 99% by mole, a fluorine content of from 15 to 75% by mole, and a molecular weight of from 1,000 to 1,000,000, provided that the monomer unit (B) is a unit derived from polymerization of any one monomer selected from the group consisting of a monomer represented by the formula CH 2 =CHCOON(R 0 ) 2 , in which the two R 0 may be the same or different and R 0 represents a hydrogen atom or an alkyl group; N-vinylcaprolactam; a monomer represented by the formula CH 2 =CR 1 CH 2 OCH 2 CR 2 =CH 2 , in which R 1 and R 2 each independently represent a hydrogen atom, a fluorine atom, or a methyl group; a monomer represented by the formula CH 2 =CHCH 2 CH(CH 3 )-R 3 , in which R 3 represents a liner alkyl group having 1 to 7 carbon atoms; and a monomer of methyl 2-fluoroacrylate.