The present invention addresses the problem of providing: a low-dose hyperphosphatemia treatment agent that can substantially reduce serum phosphorus concentration； and particles. The present invention provides a hyperphosphatemia treatment agent that contains, as an active ingredient, particles that include: a crosslinked polymer that has a substituent that includes an NRA1RA2 structure； or a salt of the crosslinked polymer. (RA1 and RA2 each independently represent a hydrogen atom, a C1-20 alkyl group, a C1-20 aminoalkyl group or a salt thereof, or the like). The average particle size of the particles is 20-150 [mu]m, and the swelling ratio of the particles is 9-16 mL/g.本發明的課題係提供可充分地降低血清磷濃度且服藥量少的高磷血症治療劑、及粒子。若依據本發明，則提供一種高磷血症治療劑，其係含有包含交聯聚合物或其鹽的粒子作為有效成分之高磷血症治療劑，該交聯聚合物具有含有NRA1RA2結構之取代基，上述粒子的平均粒徑為20~150μm，上述粒子的膨潤率為9~16mL/g(惟，RA1及RA2各自獨立地表示氫原子、碳數1~20的烷基、碳數1~20的胺基烷基或其鹽等)。
