It is an object of the present invention to provide a mutant streptavidin wherein the immunogenicity (antigenicity) in mammals of a streptavidin is reduced. The present invention provides a mutant streptavidin, which comprises an amino acid sequence in which (a) the arginine residue at position 72 is substituted with another amino acid residue, and (b) any one or more of the tyrosine residue at position 10, the tyrosine residue at position 71, the glutamic acid residue at position 89, the arginine residue at position 91, and the glutamic acid residue at position 104 are substituted with other amino acid residues, with respect to the amino acid sequence of a core streptavidin as shown in SEQ ID NO: 2, and which has decreased immunogenicity as compared with that of a wild-type streptavidin.
