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 C120 alkyl group, a C120 aminoalkyl group or a salt thereof, or the like). The average particle size of the particles is 20150 µm, and the swelling ratio of the particles is 916 mL/g.
