An object of the present invention is to provide a Fab region-binding peptide having an excellent ability for binding to a Fab region of IgG, an affinity separation matrix having the peptide as a ligand, and a method for producing a Fab region-containing protein, the method using the affinity separation matrix. Further, another object of the present invention is to provide a DNA encoding for the peptide, a vector containing the DNA, and a transformant which has been transformed by the vector. The Fab region-binding peptide according to the present invention is characterized in having a mutation at a specific site in comparison with wild-type SpG-β1.