A method of purifying a fusion protein, including: (1) adjusting the pH of an aqueous phase, containing a fusion protein which is a fusion of a protein having a self-assembly capability and a tamet protein, and which aqueous phase has a first pH, to a second pH, to obtain a remaining aqueous phase and a solid fraction containing an amount of the fusion protein; (2) separating the solid fraction from the remaining aqueous phase, to obtain a separated solid fraction; and (3) dissolving the separated solid fraction in a solution having a pH of 12 or lower but higher than the second pH by at least 0.1 pH units, wherein the protein having a self-assembly capability is a cell surface protein.