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.