PROBLEM TO BE SOLVED: To provide low molecular drug-containing nanoparticles suppressed in inducing ABC phenomenon without reducing pharmacological effect and expression of adverse effect depending on frequent administrations in nanoparticles in which a drug is enclosed and which can be used for passive targeting therapy. SOLUTION: The nanoparticles containing a negatively charged group-having low molecular drug are prepared by hydrophobing the negatively charged group-having low molecular drug with a metal ion and bringing the same to act to (a) a poly DL- or L-lactic acid or a poly (DL- or L-lactic acid/glycolic acid) copolymer and (b) one kind of copolymer selected from a (poly DL- or L-lactic acid)/(polyvinylpyrrolidone) block copolymer, a (poly DL- or L-lactic acid)/(polyacryloylmorpholine) block copolymer and a (poly DL- or L-lactic acid)/(polydimethylacrylamide) block copolymer. COPYRIGHT: (C)2011,JPO&INPIT