An A-B block copolymer having a chain A and a chain B and satisfying the following requirements to: comprising 90% by mass or more of a constituent unit derived from at least one selected from the group consisting of methacrylic acid and a methacrylate-based monomer, having a number average molecular weight of 20,000 to 60,000, and having a molecular weight distribution (weight average molecular weight/number average molecular weight) of 1.8 or less; the chain A has a constituent unit (A-i) derived from at least one selected from a monomer group (i) consisting of methyl methacrylate, benzyl methacrylate, cyclohexyl methacrylate, and t-butylcyclohexyl methacrylate, and a constituent unit (A-ii) derived from at least one selected from a monomer group (ii) consisting of 2-ethylhexyl methacrylate, lauryl methacrylate, tridecyl methacrylate, stearyl methacrylate, and behenyl methacrylate, a total content of the constituent unit (A-i) and the constituent unit (A-ii) is 80% by mass or more and a content of the constituent unit (A-ii) is 20 to 80% by mass in the chain A, and the chain A has a glass transition temperature of 30°C or lower, has a number average molecular weight of 15,000 to 50,000, and has a molecular weight distribution (weight average molecular weight/number average molecular weight) of 1.6 or less; and the chain B has a constituent unit derived from methacrylic acid, a constituent unit (B-i) derived from at least one selected from the monomer group (i) consisting of methyl methacrylate, benzyl methacrylate, cyclohexyl methacrylate, and t-butylcyclohexyl methacrylate, and a constituent unit (B-ii) derived from at least one selected from the monomer group (ii) consisting of 2-ethylhexyl methacrylate, lauryl methacrylate, tridecyl methacrylate, stearyl methacrylate, and behenyl methacrylate, a total content of the constituent unit (B-i) and the constituent unit (B-ii) in the chain is 80% by mass or more based on a total content of the constituent units other than the constituent unit, and the chain B has an acid value of 50 to 250 mgKOH/g, and has a number average molecular weight of 1,000 to 10,000. comprising 90% by mass or more of a constituent unit derived from at least one selected from the group consisting of methacrylic acid and a methacrylate-based monomer, having a number average molecular weight of 20,000 to 60,000, and having a molecular weight distribution (weight average molecular weight/number average molecular weight) of 1.8 or less; having a number average molecular weight of 20,000 to 60,000, and having a molecular weight distribution (weight average molecular weight/number average molecular weight) of 1.8 or less; the chain A has a constituent unit (A-i) derived from at least one selected from a monomer group (i) consisting of methyl methacrylate, benzyl methacrylate, cyclohexyl methacrylate, and t-butylcyclohexyl methacrylate, and a constituent unit (A-ii) derived from at least one selected from a monomer group (ii) consisting of 2-ethylhexyl methacrylate, lauryl methacrylate, tridecyl methacrylate, stearyl methacrylate, and behenyl methacrylate, a total content of the constituent unit (A-i) and the constituent unit (A-ii) is 80% by mass or more and a content of the constituent unit (A-ii) is 20 to 80% by mass in the chain A, and the chain A has a glass transition temperature of 30°C or lower, has a number average molecular weight of 15,000 to 50,000, and has a molecular weight distribution (weight average molecular weight/number average molecular weight) of 1.6 or less; and a total content of the constituent unit (A-i) and the constituent unit (A-ii) is 80% by mass or more and a content of the constituent unit (A-ii) is 20 to 80% by mass in the chain A, and the chain A has a glass transition temperature of 30°C or lower, has a number average molecular weight of 15,000 to 50,000, and has a molecular weight distribution (weight average molecular weight/number average molecular weight) of 1.6 or less; and the chain B has a constituent unit derived from methacrylic acid, a constituent unit (B-i) derived from at least one selected from the monomer group (i) consisting of methyl methacrylate, benzyl methacrylate, cyclohexyl methacrylate, and t-butylcyclohexyl methacrylate, and a constituent unit (B-ii) derived from at least one selected from the monomer group (ii) consisting of 2-ethylhexyl methacrylate, lauryl methacrylate, tridecyl methacrylate, stearyl methacrylate, and behenyl methacrylate, a total content of the constituent unit (B-i) and the constituent unit (B-ii) in the chain is 80% by mass or more based on a total content of the constituent units other than the constituent unit, and the chain B has an acid value of 50 to 250 mgKOH/g, and has a number average molecular weight of 1,000 to 10,000. a total content of the constituent unit (B-i) and the constituent unit (B-ii) in the chain is 80% by mass or more based on a total content of the constituent units other than the constituent unit, and the chain B has an acid value of 50 to 250 mgKOH/g, and has a number average molecular weight of 1,000 to 10,000.