There is provided a beraprost sodium-containing nanoparticle that contains beraprost sodium among other prostaglandin I 2 (prostacyclin) derivatives, which are therapeutic agents for pulmonary hypertension. The beraprost sodium-containing nanoparticle is obtained by making beraprost sodium hydrophobic using a metal ion and allowing the hydrophobic beraprost sodium to react with poly-L-lactic acid or a poly(L-lactic acid/glycolic acid) copolymer, and a poly-DL- or L-lactic acid-polyethylene glycol block copolymer or a poly(DL- or L-lactic acid/glycolic acid)-polyethylene glycol block copolymer. The beraprost sodium-containing nanoparticle excels in sustained release of an active ingredient, reduces a side effect, and furthermore, has an excellent drug retention in the blood. Therefore, the beraprost sodium-containing nanoparticle is quite outstanding particularly regarding the sustainability of the medicinal effect.