The present invention addresses the problem of providing a low-dose hyperphosphatemia treatment agent that can substantially reduce serum phosphorus concentration. The present invention provides a hyperphosphatemia treatment agent that contains, as an active ingredient, particles that include a prescribed crosslinked polymer (preferably crosslinked polyallylamine).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。若依據本發明,則提供含有包含指定的交聯聚合物(較佳為交聯聚烯丙胺)的粒子作為有效成分之高磷血症治療劑。