A tag database establishment section converts, based on information such as an amino acid sequence of an identified peptide and m/z of a peak, a sequence tag indicating a partial sequence and information related to the tag into a database and creates a tag database. When MS2 spectrum information is obtained by measuring a target peptide, a sequence tag acquisition section executes, on the tag database, a search based on coincidence of the m/z of the peak and the like and extracts a sequence tag with high reliability. A peptide identification section performs, based on an amino acid sequence of the sequence tag, m/z of a precursor ion, and the like, a search by a sequence tag search method taking into account a post-translational modification or a variation and identifies a peptide.