TECHNOLOGY FOCUS - POLYMERS - Preferred Components: The monomer unit (B) is: (a) a unit derived from polymerization of a monomer represented by CH2=CHCOON(R0)2, and the 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) is from 16 to 99% by mole and a ratio of the number of moles of the monomer unit (B) to the total number of moles is from 84 to 1% by mole; (b) a unit derived from polymerization of N,N-dimethylacrylamide, and the 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) is from 36 to 60% by mole and the ratio of the number of moles of the monomer unit (B) to the total number of moles is from 64 to 40% by mole; (c) a unit derived from polymerization of N-vinylcaprolactam, and the 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) is from 45 to 99% by mole and the ratio of the number of moles of the monomer unit (B) to the total number of moles is from 55 to 1% by mole; (d) a unit derived from polymerization of a monomer represented by CH2=CR1CH2OCH2CR2=CH2, and the 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) is from 10 to 99% by mole and the ratio of the number of moles of the monomer unit (B) to the total number of moles is from 90 to 1% by mole; (e) a unit derived from polymerization of a monomer represented by CH2=CHCH2CH(CH3)-R3, and the 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) is from 43 to 99% by mole and the ratio of the number of moles of the monomer unit (B) to the total number of moles is from 57 to 1% by mole; (f) a unit derived from polymerization of a monomer represented by CH2=CHCH2CH(CH3)2, and the 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) is from 43 to 70% by mole and the ratio of the number of moles of the monomer unit (B) to the total number of moles is from 57 to 30% by mole; and (g) a unit derived from polymerization of a monomer of methyl 2-fluoroacrylate, and the 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) is from 3 to 20% by mole and the ratio of the number of moles of the monomer unit (B) to the total number of moles is from 97 to 80% by mole. The membrane is a film or a coating membrane. Preferred Parameters: The fluorine-containing copolymer has an oxygen permeability coefficient of from 0 to 20 (cm3-mm)/(m2-day-atm) at 40 degrees C. The membrane has a thickness of 0.05-2000 mu m. EXAMPLE - No suitable example given.