The nanoparticles of the present invention consist of a molecular aggregate containing an amphipathic block polymer that has a hydrophilic block and a hydrophobic block. It is preferable that the amphipathic block polymer be biodegradable. The nanoparticles of the present invention are obtained by granulating into particles an amphipathic block polymer in the presence of an amino acid whose isoelectric point is 7 or less. In one embodiment, the granulation is done by bringing a solution containing an amphipathic block polymer or the dried product thereof into contact with an aqueous liquid. Granulation can be done in the presence of an amino acid by including an amino acid whose isoelectric point is 7 or less in either a solution that contains an amphipathic block polymer or in an aqueous liquid, or in both.
