A carboxyl group-containing polymer comprising: a structure unit (a) derived from an ether bond-containing monomer; a structure unit (b) derived from a sulfonic acid group-containing monomer; and a structure unit (c) derived from an acrylic acid-based monomer, the structure unit (a) represented by the formula: wherein R 0 represents a hydrogen atom or a methyl group; R 1 represents a CH 2 group, a CH 2 CH 2 group, or a direct bond; X represents a hydroxyl group or a group represented by the formula or: wherein R 2 s, which may be the same or different, represent a C2-C4 alkylene group; n represents an average addition number of moles of an oxyalkylene group (-O-R 2 -) and is 0 to 2; and R 3, R 4 and R 5 independently represent a C1-C4 alkyl group; Y represents a hydroxyl group or a group represented by the formula or; and one of X and Y is a hydroxyl group and the other is a group represented by the formula or, wherein the structure unit (a) is present at a level of from 0.5 to 15% by mass based on 100% by mass of all structure units derived from all monomers in the carboxyl group-containing polymer, the structure unit (b) represented by the formula: wherein R 6 represents a hydrogen atom or a methyl group; R 7 represents a CH 2 group, a CH 2 CH 2 group, or a direct bond; R 8 and R 9 independently represent a hydroxyl group or -SO 3 Z; Z represents a hydrogen atom, a metal atom, an ammonium group, or an organic amine group; and at least one of R 8 and R 9 is -SO 3 Z, wherein the structure unit (b) is present at a level of from 0.5 to 30% by mass based on 100% by mass of all the structure units derived from all the monomers in the carboxyl group-containing polymer, the structure unit (c) represented by the formula: wherein R 10 represents a hydrogen atom, a metal atom, an ammonium group, or an organic amine group, wherein the structure unit (c) is present at a level of from 55 to 99% by mass based on 100% by mass of all the structure units derived from all the monomers in the carboxyl group-containing polymer, and the carboxyl group-containing polymer has a weight average molecular weight determined by GPC, according to the method disclosed in the experimental section of this specification, of 20,000 to 60,000. a structure unit (a) derived from an ether bond-containing monomer; a structure unit (b) derived from a sulfonic acid group-containing monomer; and a structure unit (c) derived from an acrylic acid-based monomer, the structure unit (a) represented by the formula: wherein R 0 represents a hydrogen atom or a methyl group; R 1 represents a CH 2 group, a CH 2 CH 2 group, or a direct bond; X represents a hydroxyl group or a group represented by the formula or: wherein R 2 s, which may be the same or different, represent a C2-C4 alkylene group; n represents an average addition number of moles of an oxyalkylene group (-O-R 2 -) and is 0 to 2; and R 3, R 4 and R 5 independently represent a C1-C4 alkyl group; Y represents a hydroxyl group or a group represented by the formula or; and one of X and Y is a hydroxyl group and the other is a group represented by the formula or, wherein the structure unit (a) is present at a level of from 0.5 to 15% by mass based on 100% by mass of all structure units derived from all monomers in the carboxyl group-containing polymer, the structure unit (b) represented by the formula: wherein R 6 represents a hydrogen atom or a methyl group; R 7 represents a CH 2 group, a CH 2 CH 2 group, or a direct bond; R 8 and R 9 independently represent a hydroxyl group or -SO 3 Z; Z represents a hydrogen atom, a metal atom, an ammonium group, or an organic amine group; and at least one of R 8 and R 9 is -SO 3 Z, wherein the structure unit (b) is present at a level of from 0.5 to 30% by mass based on 100% by mass of all the structure units derived from all the monomers in the carboxyl group-containing polymer, the structure unit (c) represented by the formula: wherein R 10 represents a hydrogen atom, a metal atom, an ammonium group, or an organic amine group, wherein the structure unit (c) is present at a level of from 55 to 99% by mass based on 100% by mass of all the structure units derived from all the monomers in the carboxyl group-containing polymer, and the carboxyl group-containing polymer has a weight average molecular weight determined by GPC, according to the method disclosed in the experimental section of this specification, of 20,000 to 60,000.