A nanoparticle containing a low-molecular-weight drug having a negatively charged group is provided that is effectively targeted to an affected site, is capable of sufficiently sustained release of the drug, and has a reduced tendency to accumulate in the liver to cause reduced side effects. The nanoparticle containing a low-molecular-weight drug having a negatively charged group is obtained by hydrophobicizing the low-molecular-weight drug having a negatively charged group with a metal ion, and reacting the hydrophobicized drug with 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.
