To provide an ABA triblock copolymer containing: an A-block having, in a constitutional unit thereof, a monomer represented by the following general formula (1) or a monomer represented by the following general formula (2) and a B-block having, in a constitutional unit thereof, a hydrophilic monomer having a radical polymerizable unsaturated bond, where R 1 is a hydrogen atom or a methyl group R 2 is a C5-C18 alkylene group M is a hydrogen atom or a monovalent metal atom and X 1 is -NH- or an oxygen atom in the general formula (1), and where R 11 is a hydrogen atom or a methyl group R 12 is a C5-C18 alkylene group R 13 and R 14 are each independently a hydrogen atom, a methyl group, or an ethyl group and X 11 is -NH- or an oxygen atom in the general formula (2).
