To provide a low-cost and easy method with high yield, for separating monomer from dimer and multimer.

The method for separating polypeptide monomer from monomer and dimer or multimer, or a mixture containing both, comprises the steps of giving the mixture to a cation exchange or an anion exchange chromatography resin, wherein when the resin is for cation exchange in a buffer solution, the pH of the buffer solution is about 4-7, and when the resin is for anion exchange, the pH of the buffer solution is about 6-9; and eluting the mixture by the gradient of an elution salt of about 0-1M, wherein the monomer is separated from dimer or multimer, or both reside in the mixture.