When a liquid mixture of two samples such as biological samples labeled with stable isotopes is subjected to a relative quantitative analysis using a liquid chromatography-tandem mass spectrometry system, various constituents are simultaneously ionized. Accordingly, sufficient time required for second mass spectrometry is not ensured, whereby some ions remain unanalyzed after measurement. To address this problem, after second mass spectrometry, amino acid sequencing is performed using the analysis data of the second mass spectrometry, which enables determination on the presence/absence of a specific amino acid labeled with a stable isotope. When the specific amino acid is present, the m/z value of an isotopically-labeled-paired ion in an MS spectrum is calculated, and non-target information for use in second mass spectrometry is created using the calculated m/z information. This avoids redundant second mass spectrometry on sample components derived from the same peptide while allowing second mass spectrometry to be efficiently performed.