NOVELTY - Fluorine-containing copolymer comprises essentially of monomer units (A) of chlorotrifluoroethylene and monomer units (B), 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 as described below of from 15 to 75% by mole, and a molecular weight of from 1000 to 1000000, where monomer unit (B) is a unit derived from polymerization of any one monomer selected from specific monomers, N-vinylcaprolactam, and monomer of methyl 2-fluoroacrylate.    USE - The fluorine-containing copolymer is useful for producing coating membrane (claimed). It can also be used as a film such as a food packaging film, an electronic part packaging film, a medicament packaging film, a barrier film, a gas barrier film for liquid crystal display (LCD) or the like, which requires oxygen shielding ability, as a coating agent for light emitting diode (LED) encapsulating film, a coating film for solar cell, or the like, or as a coating agent or a film for imparting moisture shielding performance and oxygen shielding performance to a surface sheet or a back sheet of a solar battery.    ADVANTAGE - The invention provides fluorine-containing copolymer excellent in oxygen shielding performance and moisture shielding performance.    DETAILED DESCRIPTION - Fluorine-containing copolymer comprises essentially of monomer units (A) of chlorotrifluoroethylene and monomer units (B), 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 as described below of from 15 to 75% by mole, and a molecular weight of from 1000 to 1000000, where monomer unit (B) is a unit derived from polymerization of any one monomer selected from a monomer represented by CH2=CHCOON(R0)2; N-vinylcaprolactam; a monomer represented by CH2=CR1CH2OCH2CR2=CH2; a monomer represented by CH2=CHCH2CH(CH3)-R3; and a monomer of methyl 2-fluoroacrylate; and where fluorine content is a ratio of the total number of moles of fluorine atoms to the total number of moles of halogen atoms and hydrogen atoms bonded to carbon atoms of the fluorine-containing copolymer.    R0=same or different, H or alkyl group;    R1 and R2=each independently represent a hydrogen atom, a fluorine atom, or a methyl group; and    R3=a liner 1-7C alkyl group.    INDEPENDENT CLAIMS are:    (1) a membrane which is formed from a fluorine-containing copolymer above which has an oxygen permeability coefficient of from 0 to 20 (cm3-mm)/(m2-day-atm) at 40?Oc; and    (2) a process for producing a coating membrane comprising applying an organic solvent solution containing the fluorine-containing copolymer and an organic solvent onto a substrate and subsequently drying the solution to form membrane of the fluorine-containing copolymer on the substrate.