A composition which is a surface-treating composition comprising a fluorine-containing polymer having a repeating unit derived from a fluorine-containing monomer (a) and a repeating unit derived from a fluorine-free non-crosslinkable monomer (b), the fluorine-containing monomer (a) comprises - a first monomer (a1) of the formula: CH 2 =C(-Cl)-C(=O)-Y 1 -Z 1 -Rf 1 wherein Y 1 is -O- or -NH-, Z 1 is a direct bond or a divalent organic group, and Rf 1 is C 1-6 -perfluoroalkyl, and - a second monomer (a2) of the formula: CH 2 =C(-X 2 )-C(=O)-Y 2 -Z 2 -Rf 2 wherein X 2 is H or a monovalent organic group, Y 2 is -O- or -NH-, Z 2 is a direct bond or a divalent organic group, and Rf 2 is C 1-6 -perfluoroalkyl, wherein the weight ratio (a1):(a2) of the monomers (a1) and (a2) is 20:80 to 75:25; and the fluorine-free non-crosslinkable monomer (b) comprises a monomer (b1) of the formula CH 2 =CA-T wherein A is H, methyl or halogen other than F, and T is H, halogen other than F, a chain or cyclic C 1-30 -hydrocarbon group, or a chain or cyclic C 1-30 -organic group having an ester bond; a liquid aqueous medium; and a surfactant. a fluorine-containing polymer having a repeating unit derived from a fluorine-containing monomer (a) and a repeating unit derived from a fluorine-free non-crosslinkable monomer (b), the fluorine-containing monomer (a) comprises - a first monomer (a1) of the formula: CH 2 =C(-Cl)-C(=O)-Y 1 -Z 1 -Rf 1 wherein Y 1 is -O- or -NH-, Z 1 is a direct bond or a divalent organic group, and Rf 1 is C 1-6 -perfluoroalkyl, and - a second monomer (a2) of the formula: CH 2 =C(-X 2 )-C(=O)-Y 2 -Z 2 -Rf 2 wherein X 2 is H or a monovalent organic group, Y 2 is -O- or -NH-, Z 2 is a direct bond or a divalent organic group, and Rf 2 is C 1-6 -perfluoroalkyl, wherein the weight ratio (a1):(a2) of the monomers (a1) and (a2) is 20:80 to 75:25; and the fluorine-free non-crosslinkable monomer (b) comprises a monomer (b1) of the formula CH 2 =CA-T wherein A is H, methyl or halogen other than F, and T is H, halogen other than F, a chain or cyclic C 1-30 -hydrocarbon group, or a chain or cyclic C 1-30 -organic group having an ester bond; the fluorine-containing monomer (a) comprises - a first monomer (a1) of the formula: CH 2 =C(-Cl)-C(=O)-Y 1 -Z 1 -Rf 1 wherein Y 1 is -O- or -NH-, Z 1 is a direct bond or a divalent organic group, and Rf 1 is C 1-6 -perfluoroalkyl, and - a second monomer (a2) of the formula: CH 2 =C(-X 2 )-C(=O)-Y 2 -Z 2 -Rf 2 wherein X 2 is H or a monovalent organic group, Y 2 is -O- or -NH-, Z 2 is a direct bond or a divalent organic group, and Rf 2 is C 1-6 -perfluoroalkyl, - a first monomer (a1) of the formula: CH 2 =C(-Cl)-C(=O)-Y 1 -Z 1 -Rf 1 wherein Y 1 is -O- or -NH-, Z 1 is a direct bond or a divalent organic group, and Rf 1 is C 1-6 -perfluoroalkyl, and Y 1 is -O- or -NH-, Z 1 is a direct bond or a divalent organic group, and Rf 1 is C 1-6 -perfluoroalkyl, and - a second monomer (a2) of the formula: CH 2 =C(-X 2 )-C(=O)-Y 2 -Z 2 -Rf 2 wherein X 2 is H or a monovalent organic group, Y 2 is -O- or -NH-, Z 2 is a direct bond or a divalent organic group, and Rf 2 is C 1-6 -perfluoroalkyl, X 2 is H or a monovalent organic group, Y 2 is -O- or -NH-, Z 2 is a direct bond or a divalent organic group, and Rf 2 is C 1-6 -perfluoroalkyl, wherein the weight ratio (a1):(a2) of the monomers (a1) and (a2) is 20:80 to 75:25; and the fluorine-free non-crosslinkable monomer (b) comprises a monomer (b1) of the formula CH 2 =CA-T wherein A is H, methyl or halogen other than F, and T is H, halogen other than F, a chain or cyclic C 1-30 -hydrocarbon group, or a chain or cyclic C 1-30 -organic group having an ester bond; a liquid aqueous medium; and a surfactant. <EOS>