The present invention relates to a graft copolymer of a poly(amino acid) or a salt thereof and a hydrophobic primary amine compound or a salt thereof ( e.g., a graft copolymer (³-PGA-PAE) of poly(³-glutamic acid) (³-PGA) and phenylalanine ethyl ester (PAE)), an ionized graft copolymer of a poly(amino acid) or a salt thereof and a hydrophobic primary amine compound or a salt thereof, nanoparticles containing the ionized graft copolymer, and a production method thereof. The nanoparticles acquired in this way are useful as an adjuvant for producing a vaccine.