PROBLEM TO BE SOLVED: To provide nanoparticles containing beraprost sodium among prostaglandin I(prostacyclin) derivatives which are therapeutic agents of pulmonary hypertension.SOLUTION: The nanoparticle containing beraprost sodium, which is obtained by hydrophobizing beraprost sodium by metal ion, and by allowing the hydrophobized product to act on poly L-lactic acid or poly(L-lactic acid/glycolic acid) copolymer, and poly(DL- or L-lactic acid)-polyethylene glycol block copolymer or poly(DL- or L-lactic acid/glycolic acid)-polyethylene glycol block copolymer, is excellent in sustained release property of an active ingredient, further excellent in retainability in blood, reduces side effects, and especially has extremely remarkable sustainability in medicinal effect.