Disclosed is a method for purifying a target protein, comprising steps of: bringing a fusion protein containing an amino acid sequence of a peptide tag, an amino acid sequence of a cleavable site of a protease and an amino acid sequence of a target protein, into contact with the protease in a solution, to cleave the peptide tag from the fusion protein; and bringing the solution containing the peptide tag, the target protein and the protease into contact with an ion exchange resin to separate the target protein and the peptide tag, thereby acquiring a solution containing the target protein, wherein, in the fusion protein, the amino acid sequence of a cleavable site of the protease exists between the amino acid sequence of the peptide tag and the amino acid sequence of the target protein, and the peptide tag is polyanionic or polycationic.