A
fluorine-containing
copolymer, which is obtained by copolymerizing, as
copolymer units, (A) the general formula: CnF2n+1(CH2CF2)a(CF2CF2)b(CH2CH2)cOCOCR=CH2 wherein R is a
hydrogen atom or a
methyl group, n is 1 to 6, a is an integer of 1 to 4, b is 1 to 3, and c is 1 to 3; (B) the general formula: CH2=CRCOOR1 wherein R is a
hydrogen atom or a
methyl group, and R1 is a C1-C30 linear, branched, or
cyclic hydrocarbon group that may contain an
oxygen atom; (C) the general formula: CH2=CRCOO(H)p(R2Y)q wherein R is a
hydrogen atom or a
methyl group, p and q are 0 or 1, p + q is 1, R2 is a C1-C30 linear, branched, or cyclic
divalent hydrocarbon group that may have an
oxygen atom, and Y is a
phosphate group; and (D) the general formula: CH2 =CRCOOR3 OCOCR=CH2 wherein R is a
hydrogen atom or a methyl group, R3 is a C1-C30 linear, branched, or cyclic
divalent hydrocarbon group that may contain an
oxygen atom, at a ratio of 1 to 50 wt.% of the component (B), 0.01 to 5 wt.% of the component (C) and 0.01 to 2 wt.% of the component (D). The
fluorine-containing
copolymer gives a
substrate surface modied film having excellent water and oil repellency,
rust resistance, and the like.