PROBLEM TO BE SOLVED: To provide a method for incorporating one or more acyl groups to a peptide or a protein.SOLUTION: The method includes reacting (a) a peptide (or a protein) having at least one free amino group with an acylation agent represented by formula (I) under a basic condition in a mixture of an aprotic polar solvent and water, and saponifying the peptide ester group acylated in the above under a basic condition.