A guanidino group in an amino acid or a peptide can be protected with a specific group, i.e. pentamethylbenzenesulfonyl, 2,4,6-trimethoxybenzenesulfonyl, 4-methoxy-2,3,5,6-tetramethylbenzenesulfonyl, 4-methoxy-2,6-dimethylbenzenesulfonyl or 4-methoxy-2,3,6-trimethylbenzenesulfonyl, and said group may easily be removed without affecting the amino acid or the peptide to be derived from the protected amino acid or peptide. Thus, the present invention is useful in the synthesis of peptides containing the guanidino group.