PROBLEM TO BE SOLVED: To provide a boron compound excellent in retention in blood, tumor accumulation properties and/or the content of the boron compound loaded into each structure or a polymer micelle and capable of enhancing a therapeutic effect by a neutron beam capture therapy.SOLUTION: The boron compound is a polymeric boron compound formed by covalently bonding a boron ion cluster through a chloro methylene group of a polyethylene-glycols b-poly chloro methylene styrene (PEG-b-PCMA) block copolymer, and a polyion composite of the copolymer and a specified polycation chargeable compound.