A copolymer comprising the following polymerized units (a), polymerized units (b), polymerized units and polymerized units: Polymerized units (a): polymerized units based on a monomer represented by the following formula: Z-Y-X (in the formula, Z is a C 4-6 polyfluoroalkyl group or a group represented by the following formula, Y is a bivalent organic group or a single bond, and X is any one of groups represented by the following formulae to: C i F 2i+1 O(CFX 1 CF 2 O) j CFX 2 - (wherein i is an integer of from 1 to 6, j is an integer of from 0 to 10, and each of X 1 and X 2 is a fluorine atom or a trifluoromethyl group), -CR=CH 2 -C OCR=CH 2 -OC CR=CH 2 -OCH 2 -φ-CR=CH 2 -OCH=CH 2 (wherein R is a hydrogen atom, a methyl group or a halogen atom, and φ is a phenylene group), Polymerized units (b): polymerized units based on a linear or branched alkyl (meth)acrylate wherein the alkyl moiety has from 6 to 18 carbon atoms, (provided that when in the polymerized units (a), X in the formula is a group represented by any one of the formulae to wherein R is a group being a methyl group or a halogen atom, the polymerized units (b) are polymerized units based on an alkyl acrylate, and when in the polymerized units (a), X in the formula is a group represented by any one of the formulae to wherein R is a group being a hydrogen atom, or a group represented by the formula, the polymerized units (b) are polymerized units based on an alkyl methacrylate), Polymerized units: polymerized units based on a monomer having a hydroxy group without having a polyfluoroalkyl group, and Polymerized units: polymerized units based on a monomer having a functional group which is reactive with the hydroxy group in the polymerized units to form a crosslinked structure. Polymerized units (a): polymerized units based on a monomer represented by the following formula: Z-Y-X (in the formula, Z is a C 4-6 polyfluoroalkyl group or a group represented by the following formula, Y is a bivalent organic group or a single bond, and X is any one of groups represented by the following formulae to: C i F 2i+1 O(CFX 1 CF 2 O) j CFX 2 - (wherein i is an integer of from 1 to 6, j is an integer of from 0 to 10, and each of X 1 and X 2 is a fluorine atom or a trifluoromethyl group), -CR=CH 2 -C OCR=CH 2 -OC CR=CH 2 -OCH 2 -φ-CR=CH 2 -OCH=CH 2 (wherein R is a hydrogen atom, a methyl group or a halogen atom, and φ is a phenylene group), Polymerized units (b): polymerized units based on a linear or branched alkyl (meth)acrylate wherein the alkyl moiety has from 6 to 18 carbon atoms, (provided that when in the polymerized units (a), X in the formula is a group represented by any one of the formulae to wherein R is a group being a methyl group or a halogen atom, the polymerized units (b) are polymerized units based on an alkyl acrylate, and when in the polymerized units (a), X in the formula is a group represented by any one of the formulae to wherein R is a group being a hydrogen atom, or a group represented by the formula, the polymerized units (b) are polymerized units based on an alkyl methacrylate), Polymerized units: polymerized units based on a monomer having a hydroxy group without having a polyfluoroalkyl group, and Polymerized units: polymerized units based on a monomer having a functional group which is reactive with the hydroxy group in the polymerized units to form a crosslinked structure.