A peptide is cleaved at various bonding sites into oligopeptides or similar fragments by digestion using proteinase K (S3). The obtained fragments are separated according to their kinds by reversed-phase chromatography and fractionated (S4), and each fragment is subjected to mass spectrometry to determine its mass (S6). For each peptide fragment, an amino-acid composition is calculated from the measured mass, and amino-acid sequence candidates are deduced from that composition. The amino-acid sequence candidates of the other peptide fragments are searched for a fragment having an overlapping portion available for combining two peptide fragments to obtain amino-acid sequence candidates of the original peptide (S7). The masses of the amino-acid sequence candidates are compared with a measured mass derived from a result of mass spectrometry of the original peptide to select a correct sequence (S6 and S8).