The present invention provides a star-shaped acrylic block polymer having a star-shaped structure in which at least three chain-like polymer parts extend radially around a mercapto group as a center, characterized in that 30 to 99.9 mass % based on the whole structural units of the star-shaped acrylic block polymer is composed of a (meth)acrylic acid alkyl ester having 7 to 17 carbons structural unit, and at least one of the above-mentioned chain-like polymer parts is in the form of a copolymer structure of polymerizable monomers including a (meth)acrylic acid alkyl ester having 7 to 17 carbons and a weakly basic monomer； as well as a medical patch comprising an adhesive composition wherein the adhesive composition comprises a salt of a weakly basic drug and the star-shaped acrylic block polymer as an adhesive, which shows a high efficacy of drug, is free of lowering in adhesive properties of an adhesive base due to additives, and also shows a low skin irritation.本發明係提供一種星形丙烯酸系嵌段聚合物，其係持有至少3條鏈狀聚合物部分以巰基為中心延伸成放射狀之星形結構，該星形丙烯酸系嵌段聚合物的所有結構單元之30~99.9質量%為碳數7~17的(甲基)丙烯酸烷酯結構單元，至少1條前述鏈狀聚合物部分具有包含碳數7~17的(甲基)丙烯酸烷酯及弱鹼性單體之聚合性單體的共聚物結構；以及一種醫療用貼附劑，其係包含黏著劑組成物，該黏著劑組成物包含鹼性藥物之鹽及作為黏著劑之該星形丙烯酸系嵌段聚合物，其係顯示出較高的藥效，無添加劑所引發之黏著基劑的黏著物性降低且皮膚刺激性較低。
