To provide a method for efficiently determining a sequence of amino acids of peptide.

The method for determining the sequence of amino acids of peptide includes (1) a process of deuterium labeling an N terminal amino group of the peptide, (2) a process of obtaining a mass spectrum of fragment ions for the deuterium labeled peptide, and (3) a process of specifying data originating from the N terminal amino group of the peptide and data originating from a C terminal carboxyl group from the mass spectrum data obtained in the process (2), analyzing a sequence of amino acids of each data, and determining the sequence of amino acids of the peptide.