Described is a labeling technique which can facilitate the metabolism in the liver after administration to patients without the reduction in the antibody function, thereby reducing accumulation of radionuclides in an organ such as the liver, and a modified antibody containing an IgG antibody and an IgG-binding peptide bound to the IgG antibody. The IgG-binding peptide has an amino acid sequence consisting of 13 to 17 amino acid residues, such as GPDCAYH(Xaa1)GELVWCTFH (SEQ ID NO: 2) wherein Xaa1 represents a lysine residue, a cysteine residue, an aspartic acid residue, a glutamic acid residue, 2-aminosuberic acid, or diaminopropionic acid, and a compound represented by the following formula (II-1) is linked at a position of the lysine residue via a modification linker to the N terminus of the IgG-binding peptide.