The objective of the present invention is to obtain an MS2 spectrum for each of a plurality of different ion species even when their m/z values are extremely close to each other and prevent separate setting of each ion species as the precursor ion. In the vicinity of the target m/z, a precursor-ion selection window covering a predetermined m/z range (2×ΔM) is gradually shifted in predetermined steps (Δm) to define a plurality of windows as the condition of the precursor-ion selection. When an MS2 analysis is performed on the same sample for each window, the intensities of the product-ion peaks which appear on the MS2 spectrum change with the change in the central m/z value of the window. From this intensity change, which of the plurality of ion species selected as the precursor ion is the origin of each product ion is determined. Based on the result of this determination, the product ions are sorted out and an MS2 spectrum is reconstructed for each ion species.