1. A therapeutic agent for hyperphosphatemia which comprises, as an active ingredient, a
particle comprising a crosslinked polymer having a substituent containing a NRA1RA2 structure,
or a salt thereof, wherein the particle has an average particle diameter of 20 to 150 µm and a
swelling rate of 8 to 20 mL/g,
wherein:
RA1 and RA2 each independently represent a hydrogen atom, an alkyl group having 1 to 20 carbon atoms, an aminoalkyl group having 1 to 20 carbon atoms or a salt thereof, an alkylaminoalkyl group having 2 to 20 carbon atoms or a salt thereof, a dialkylaminoalkyl group having 3 to 20 carbon atoms or a salt thereof, a trialkylammoniumalkyl group having 4 to 20 carbon atoms, an alkylcarbonyl group having 1 to 20 carbon atoms, a carboxyalkyl group having 1 to 20 carbon atoms, or a hydroxyalkyl group having 1 to 20 carbon atoms;
the average particle diameter is calculated as a volume average particle diameter by converting an area of 1000 or more imaged particles dispersed in water in an optical microscope photograph to diameters and using the diameters; and
the swelling rate is calculated by dividing, by a mass of the particle before swelling, a volume of the swollen particle which is obtainable by repeating shaking and 1-hour or longer still standing 20 or more times in an aqueous solution containing 2.2% by mass of sodium 2-morpholinoethanesulfonate and 0.5% by mass of sodium chloride and having a pH of 6.3 at 20°C.
2. The therapeutic agent for hyperphosphatemia according to claim 1, wherein the particle
is a globule.
3. The therapeutic agent for hyperphosphatemia according to claim 1 or 2, wherein the
particle has an outer shell part and a central part and the central part has a lower degree of
crosslinking than the outer shell part.
4. The therapeutic agent for hyperphosphatemia according to any one of claims 1 to 3,
wherein the particle has an outer shell part and a central part and the central part has a smaller
crosslinked polymer abundance than the outer shell part.

5. The therapeutic agent for hyperphosphatemia according to any one of claims 1 to 4,
wherein when a free induction attenuation signal obtained in pulse NMR is subjected to
waveform separation by subtracting components in the descending order in terms of spin-spin
relaxation time T2 using a least-square method, whereby the particle is divided into three
components: a non-restrained part, a semi-restrained part and a restrained part in the descending
order in terms of spin-spin relaxation time, the particle has a proportion of a semi-restrained part
of 25 to 70%.
6. The therapeutic agent for hyperphosphatemia according to any one of claims 1 to 5, wherein when a free induction attenuation signal obtained in pulse NMR is subjected to waveform separation by subtracting components in the descending order in terms of spin-spin relaxation time T2 using a least-square method, whereby the particle is divided into three components: a non-restrained part, a semi-restrained part and a restrained part in the descending order in terms of spin-spin relaxation time, the particle has a proportion of the restrained part of 30 to 70%.
7. The therapeutic agent for hyperphosphatemia according to any one of claims 1 to 6 wherein a phosphate adsorption capacity is 6.0 to 10.0 mmol/g,
wherein the phosphate adsorption capacity is calculated by: when 30 mg of particles is mixed and stirred at 37°C for 1 hour in 20 mL of aqueous solution containing 2.2% by mass of sodium morpholinoethanesulfonate, 0.47% by mass of sodium chloride and 0.24% by mass of phosphate and having a pH of 6.4, quantifying phosphate concentrations in a supernatant before and after mixing by ICP emission spectrochemical analysis; dividing a decrease thereof by a mass of the particles; and correcting by use of a loss on drying.
8. The therapeutic agent for hyperphosphatemia according to any one of claims 1 to 7 wherein
an amine value is 11.0 to 17.5 mmol/g,
wherein the amine value is calculated by: treating particles dispersed in ultrapure water with 5 N hydrochloric acid; quantifying an amino group by conducting neutralization titration with 0.1 N sodium hydroxide aqueous solution; dividing by a mass of the particles; and correcting by use of a loss on drying.
9. The therapeutic agent for hyperphosphatemia according to any one of claims 1 to 8,

wherein the particle is obtained through a crosslinking reaction caused by adding a crosslinking agent to an emulsion prepared by emulsifying a polymer having a substituent containing a NRA1RA2 structure or a salt thereof. 10 The therapeutic agent for hyperphosphatemia according to any one of claims 1 to 9,
wherein:
the particle is obtained through a crosslinking reaction caused by adding a crosslinking agent to an emulsion prepared by emulsifying a polymer having a substituent containing a NRA1RA2 structure or a salt thereof;
the emulsion is obtained by mixing a first solution containing the polymer or a salt thereof, and a hydrophilic solvent and having a viscosity of 10 to 2000 mPa-s with a second solution containing a hydrophobic solvent and having a viscosity of 1 to 100 mPa-s; and
a ratio of the viscosity of the first solution to the viscosity of the second solution is within 0.1:1 to 300:1.
11. The therapeutic agent for hyperphosphatemia according to claim 10, wherein the first solution has a viscosity of 10 to 1500 mPa-s.
12. The therapeutic agent for hyperphosphatemia according to claim 10 or 11, wherein the ratio of the viscosity of the first solution to the viscosity of the second solution is within 0.2:1 to 100:1.
13. The therapeutic agent for hyperphosphatemia according to any one of claims 10 to 12, wherein the second solution contains an emulsifier having a weight average molecular weight or a number average molecular weight of 2000 or more.
14. The therapeutic agent for hyperphosphatemia according to claim 13, wherein the
emulsifier contains a saccharide.
15. The therapeutic agent for hyperphosphatemia according to claim 13 or 14, wherein the emulsifier contains cellulose ether.
16. The therapeutic agent for hyperphosphatemia according to any one of claims 1 to 15, wherein the crosslinked polymer having a substituent containing a NRA1RA2 structure is a crosslinked polymer having at least a repeating unit A represented by the following formula (1-1) or (1-2),

viscosity of 1 to 100 mPa-s, wherein a ratio of the viscosity of the first solution to the viscosity of the second solution is within 0.1:1 to 300:1,
wherein RA1 and RA2 each independently represent a hydrogen atom, an alkyl group having 1 to 20 carbon atoms, an aminoalkyl group having 1 to 20 carbon atoms or a salt thereof, an alkylaminoalkyl group having 2 to 20 carbon atoms or a salt thereof, a dialkylaminoalkyl group having 3 to 20 carbon atoms or a salt thereof, a trialkylammoniumalkyl group having 4 to 20 carbon atoms, an alkylcarbonyl group having 1 to 20 carbon atoms, a carboxyalkyl group having 1 to 20 carbon atoms, or a hydroxyalkyl group having 1 to 20 carbon atoms.
19. The particle according to claim 18, wherein the particle has an average particle diameter
of 20 to 150 (im and a swelling rate of 8 to 20 mL/g,
wherein:
the average particle diameter is calculated as a volume average particle diameter by converting an area of 1000 or more imaged particles dispersed in water in an optical microscope photograph to diameters and using the diameters; and
the swelling rate is calculated by dividing, by a mass of the particle before swelling, a volume of the swollen particle which is obtainable by repeating shaking and 1-hour or longer still standing 20 or more times in an aqueous solution containing 2.2% by mass of sodium 2-morpholinoethanesulfonate and 0.5% by mass of sodium chloride and having a pH of 6.3 at 20°C.
20. A therapeutic agent for hyperphosphatemia comprising, as an active ingredient, a particle according to claim 18 or 19.
21. A particle obtained through a crosslinking reaction caused by adding a crosslinking agent to an emulsion prepared by mixing a first solution containing polyallylamine or a salt thereof, and a hydrophilic solvent with a second solution containing cellulose ether and a hydrophobic solvent.
22. A therapeutic agent for hyperphosphatemia comprising, as an active ingredient, a particle according to claim 21.

23. A therapeutic agent for hyperphosphatemia comprising, as an active ingredient, a particle
containing a crosslinked polymer having a substituent containing a NRA1RA2 structure or a salt
thereof,
wherein when a free induction attenuation signal obtained in pulse NMR is subjected to waveform separation by subtracting components in the descending order in terms of spin-spin relaxation time T2 using a least-square method, whereby the particle is divided into three components: a non-restrained part, a semi-restrained part and a restrained part in the descending order in terms of spin-spin relaxation time, the particle has a proportion of the semi-restrained part of 25 to 70%,
wherein RA1 and RA2 each independently represent a hydrogen atom, an alkyl group having 1 to 20 carbon atoms, an aminoalkyl group having 1 to 20 carbon atoms or a salt thereof, an alkylaminoalkyl group having 2 to 20 carbon atoms or a salt thereof, a dialkylaminoalkyl group having 3 to 20 carbon atoms or a salt thereof, a trialkylammoniumalkyl group having 4 to 20 carbon atoms, an alkylcarbonyl group having 1 to 20 carbon atoms, a carboxyalkyl group having 1 to 20 carbon atoms, or a hydroxyalkyl group having 1 to 20 carbon atoms.
24. The therapeutic agent for hyperphosphatemia according to claim 23, wherein the proportion of the restrained part is 30 to 70%.
25. A particle which comprises a crosslinked polymer having a substituent containing a NRA1RA2 structure or a salt thereof,
wherein when a free induction attenuation signal obtained in pulse NMR is subjected to waveform separation by subtracting components in the descending order in terms of spin-spin relaxation time T2 using a least-square method, whereby the particle is divided into three components: a non-restrained part, a semi-restrained part and a restrained part in the descending order in terms of spin-spin relaxation time, the particle has a proportion of the semi-restrained part of 25 to 70%,
wherein RA1 and RA2 each independently represent a hydrogen atom, an alkyl group having 1 to 20 carbon atoms, an aminoalkyl group having 1 to 20 carbon atoms or a salt thereof, an alkylaminoalkyl group having 2 to 20 carbon atoms or a salt thereof, a dialkylaminoalkyl group having 3 to 20 carbon atoms or a salt thereof, a trialkylammoniumalkyl group having 4

to 20 carbon atoms, an alkylcarbonyl group having 1 to 20 carbon atoms, a carboxyalkyl group
having 1 to 20 carbon atoms, or a hydroxyalkyl group having 1 to 20 carbon atoms.
26. The particle according to claim 25, wherein the proportion of the restrained part is 30 to
70%.