A technology which enables a high yield and convenient recovery of a protein and also enables a global protein purification is provided.A protein tag comprising an amino acid sequence of the full length or a part of MafG protein or an amino acid sequence in which amino acids serving as protease cleavage sites were inserted into the former amino acid sequence is provided. Since this protein tag can impart a high insolubility attributable to MafG protein to a protein to be tagged thereby insolubilizing the tagged protein, the tagged protein can be recovered into an insoluble fraction at a high yield.