Provided is a technique for accurately identifying a peptide even if the peptide cannot be identified by MS/MS ion search or de novo sequencing. The technique uses an MSn spectrum for a low m/z precursor ion. Such a spectrum contains a rather small amount of information, but this information is highly reliable when used for deducing an amino acid sequence by de novo sequencing. Accordingly, an MSn spectrum for a precursor ion having an m/z value of 500 or less is intentionally obtained, and de novo sequencing is performed on this spectrum to deduce the amino acid sequence covering the range from the m/z of the precursor ion and m/z=0. A highly reliable sequence tag can be generated from the deduced amino acid sequence, the m/z of the precursor ion of the MSn analysis, and the m/z of the precursor ion of the MS2 analysis.