A copolymer comprising: a residue unit A represented by the general formula; and a residue unit B represented by the general formula: wherein R 1 and R 2 each independently represent hydrogen provided that the case where R 1 and R 2 are both hydrogen is excluded, a cyano group, an ester group (-C(=O)OX 1), an amide group (-C(=O)N(X 2)(X 3)), or an acyl group (-C(=O)X 4), where X 1 to X 3 each independently represent a linear alkyl group having 1 to 12 carbon atoms, a branched alkyl group having 1 to 12 carbon atoms, or a cyclic alkyl group having 3 to 6 carbon atoms, and X 4 represents a linear alkyl group having 1 to 12 carbon atoms, a branched alkyl group having 1 to 12 carbon atoms, or a cyclic group having 3 to 14 carbon atoms; R 3 to R 7 each independently represent hydrogen, a linear alkyl group having 1 to 12 carbon atoms, a branched alkyl group having 1 to 12 carbon atoms, a cyclic group having 3 to 14 carbon atoms, a halogen, a hydroxy group, a carboxy group, a nitro group, a cyano group, an alkoxy group (-OX 5), an ester group (-C(=O)OX 6), an amide group (-C(=O)N(X 7)(X 8)), an acyl group (-C(=O)X 9), an amino group (-N(X 10)(X 11)), or a sulfonyl group (-SOOX 12), where X 5 to X 8 each independently represent a linear alkyl group having 1 to 12 carbon atoms, a branched alkyl group having 1 to 12 carbon atoms, or a cyclic alkyl group having 3 to 6 carbon atoms, and X 9 to X 12 each independently represent hydrogen, a linear alkyl group having 1 to 12 carbon atoms, a branched alkyl group having 1 to 12 carbon atoms, or a cyclic alkyl group having 3 to 6 carbon atoms; any one or more of R 3 to R 7 is an hydroxy group; and adjacent substituents among R 3 to R 7 may form a fused ring structure each other, wherein R 8 represents an m-membered heterocyclic residue containing one or more heteroatoms or a 5-membered or 6-membered ring residue containing no heteroatom, and m represents an integer of 5 to 10; and the m-membered heterocyclic residue, the 5-membered ring residue, and the 6-membered ring residue may form a fused ring structure with another cyclic structure. a residue unit A represented by the general formula; and a residue unit B represented by the general formula: wherein R 1 and R 2 each independently represent hydrogen provided that the case where R 1 and R 2 are both hydrogen is excluded, a cyano group, an ester group (-C(=O)OX 1), an amide group (-C(=O)N(X 2)(X 3)), or an acyl group (-C(=O)X 4), where X 1 to X 3 each independently represent a linear alkyl group having 1 to 12 carbon atoms, a branched alkyl group having 1 to 12 carbon atoms, or a cyclic alkyl group having 3 to 6 carbon atoms, and X 4 represents a linear alkyl group having 1 to 12 carbon atoms, a branched alkyl group having 1 to 12 carbon atoms, or a cyclic group having 3 to 14 carbon atoms; R 3 to R 7 each independently represent hydrogen, a linear alkyl group having 1 to 12 carbon atoms, a branched alkyl group having 1 to 12 carbon atoms, a cyclic group having 3 to 14 carbon atoms, a halogen, a hydroxy group, a carboxy group, a nitro group, a cyano group, an alkoxy group (-OX 5), an ester group (-C(=O)OX 6), an amide group (-C(=O)N(X 7)(X 8)), an acyl group (-C(=O)X 9), an amino group (-N(X 10)(X 11)), or a sulfonyl group (-SOOX 12), where X 5 to X 8 each independently represent a linear alkyl group having 1 to 12 carbon atoms, a branched alkyl group having 1 to 12 carbon atoms, or a cyclic alkyl group having 3 to 6 carbon atoms, and X 9 to X 12 each independently represent hydrogen, a linear alkyl group having 1 to 12 carbon atoms, a branched alkyl group having 1 to 12 carbon atoms, or a cyclic alkyl group having 3 to 6 carbon atoms; any one or more of R 3 to R 7 is an hydroxy group; and adjacent substituents among R 3 to R 7 may form a fused ring structure each other, wherein R 8 represents an m-membered heterocyclic residue containing one or more heteroatoms or a 5-membered or 6-membered ring residue containing no heteroatom, and m represents an integer of 5 to 10; and the m-membered heterocyclic residue, the 5-membered ring residue, and the 6-membered ring residue may form a fused ring structure with another cyclic structure.