An analyte peptide is selectively degraded sequentially by using an alkanoic anhydride (S101). The original peptide and a series of degradation reaction products having peptide in which one or more C-terminal-sided amino acids are deleted, are subjected to a certain posttreatment (S102). The molecular weight of the reaction products is measured by mass spectrometry (S103). And, the amino acid sequence of the original peptide from C-terminal is determined, based on the molecular weight obtained by mass spectrometry (S104).