A polyion complex (PIC) or a PIC nanoparticle that may be easily prepared, and that is finally disappeared in vivo due to its suitable biodegradability while exhibiting high stability in vivo , an immunotherapy agent comprising the PIC nanoparticle to which various antigen proteins or peptides may be easily conjugated or incorporated and/or which may be easily mixed with the antigen proteins or peptides, as well as a process for preparing thereof are provided. Specifically, a polyion complex (PIC) comprising a hydrophobized poly(acidic amino acid) and a basic polypeptide, a nanoparticle thereof having a particle shape, an immunotherapy agent comprising the PIC nanoparticle, as well as a process for preparing the PIC, comprising steps of introducing a hydrophobic amino acid to a poly(acidic amino acid) to prepare a hydrophobized poly(acidic amino acid), and dissolving the hydrophobized poly(acidic amino acid) prepared to a buffer, and it is mixed with a basic polypeptide dissolved in a buffer.
