The analyst previously enters the mass of a fragment that desorbs in the first dissociation with other analysis conditions, as the precursor ion selection reference for the second dissociation through the input unit 25. When the automatic analysis is started, the controller unit 21 sequentially performs the MS1 analysis, MS2 analysis and MS3 analysis. In the course of these analyses, the data processing unit 23 determines the valence of each ion species corresponding to the peaks appearing in the mass spectrum obtained by the MS1 analysis. In addition, after the MS2 analysis, the data processing unit 23 searches for the ion species in conformity with the selection reference in consideration of the determined valence, among the ion species corresponding to the peaks appearing in the mass spectrum by the MS2 analysis. The selected ion is determined as the precursor ion for the second dissociation in the MS3 analysis. In this manner, regardless of the valence of the target ion, the precursor ions to be selected and dissociated in each stage of the MSn analysis are automatically selected according to the mass of the fragment desorbed in the dissociation in the previous stage. Therefore, the analytical efficiency is improved and the highly accurate chemical structure information can be obtained.