Disclosed is a protein aqueous suspension preparation containing a protein and a polyamino acid, the protein and the polyamino acid having a surface charge in a buffer and forming a complex suspended in the buffer, wherein the absolute value of the difference between pH of the buffer and isoelectric point pI of the protein is in the range of from 0.5 to 4.0. Also disclosed are a method of preparing a protein aqueous suspension preparation and a prefilled syringe containing a concentrated protein aqueous suspension preparation. The protein can exhibit at least one of shaking stress resistance, fluidity enhancement, oxidation resistance, thermal stability, and aggregation inhibitory properties.
