A method for purifying a target protein, comprising steps of introducing a vector comprising a polynucleotide encoding a fusion protein into a host cell, wherein the fusion protein comprises 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 expressing the fusion protein in the host cell to prepare a sample solution comprising the fusion protein and contaminant proteins, wherein the salt concentration of the sample solution is 50 mM or more and 500 mM or less, removing contaminant proteins from the sample solution by: passing the sample solution through an anion exchange resin to obtain (i) an anion exchange resin to which the fusion protein is bound and (ii) a flow-through fraction comprising the contaminant proteins, removing the flow-through fraction, and eluting the fusion protein from the anion exchange resin by passing a buffer with a salt concentration of 600 mM or more through the anion exchange resin, bringing the fusion protein into contact with the protease in a solution, to cleave the peptide tag from the fusion protein, diluting the solution comprising the peptide tag, the target protein and the protease to obtain a salt concentration of 100 mM or more and 500 mM or less, and bringing the diluted solution comprising the peptide tag, the target protein and the protease into contact with an anion exchange resin to separate the target protein from the protease and the peptide tag, thereby obtaining a solution comprising the target protein, wherein in the fusion protein, the amino acid sequence of a cleavable site is located between the amino acid sequence of the peptide tag and the amino acid sequence of the target protein, the peptide tag is polyanionic the isoelectric point of the fusion protein is less than 5, the isoelectric point of the target protein is 5 or more, and the protease is fused with the peptide tag but does not have an amino acid sequence of a cleavable site of the protease and the isoelectric point of protease fused with the peptide tag is such that it binds to the ion exchange resin; and wherein in the second bringing step, the diluted solution comprising the polyanionic peptide tag, the target protein and the protease fused to the peptide tag is passed through the anion exchange resin, the polyanionic peptide tag and the protease fused to the peptide tag are bound to the anion exchange resin, and a flow-through fraction comprising the target protein that does not bind to the anion exchange resin is obtained. passing the sample solution through an anion exchange resin to obtain (i) an anion exchange resin to which the fusion protein is bound and (ii) a flow-through fraction comprising the contaminant proteins, removing the flow-through fraction, and eluting the fusion protein from the anion exchange resin by passing a buffer with a salt concentration of 600 mM or more through the anion exchange resin, the diluted solution comprising the polyanionic peptide tag, the target protein and the protease fused to the peptide tag is passed through the anion exchange resin, the polyanionic peptide tag and the protease fused to the peptide tag are bound to the anion exchange resin, and a flow-through fraction comprising the target protein that does not bind to the anion exchange resin is obtained.