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 R1 is a hydrogen atom or a methyl group; R2 is a C5-C18 alkylene group; M is a hydrogen atom or a monovalent metal atom; and X1 is —NH— or an oxygen atom in the general formula (1), and where R11 is a hydrogen atom or a methyl group; R12 is a C5-C18 alkylene group; R13 and R14 are each independently a hydrogen atom, a methyl group, or an ethyl group; and X11 is —NH— or an oxygen atom in the general formula (2).