A method of mass spectrometry. The method starts with gaining a total ion chromatogram by a mass spectrometer. An accumulation mass spectrum indicating vicinities of a desired peak top of the total ion chromatogram is displayed. The operator selects a desired mass peak from the displayed accumulation mass spectrum. Another mass chromatogram that is coincident in elution start time, maximum elution time, and elution end time with the mass chromatogram of the selected mass peak are extracted and selected out as a mass peak. Thus, a mass spectrum of a single component is reconstructed.