Provided is a method of removing a protein aggregate which can obtain a useful protein monomer in a high yield and a high purity as a raw material of a pharmaceutical product or the like. The method of removing a protein aggregate includes: a step of making the monomer of a protein and the aggregate of proteins adsorb to the column by making a solution containing a monomer of a protein and an aggregate of proteins pass through a column containing a hard porous polymer self-supporting structure to which a strong cation exchange group is fixed and a step of making a mobile phase consisting of a mixed solution of a buffer solution and an ionic buffer solution pass through the column to which the monomer of a protein and the aggregate of proteins are adsorbed, to selectively elute the monomer of a protein.
