There are provided a mass spectrometer and a mass spectrometry method which can realize shortening of the measurement time and reduction of the consumption of a sample. Ions, in which the intensity distribution forms a peak waveform at both of each retention time and each mass-to-charge ratio (peaks P11, P21, P22 and P32) are detected as MS/MS precursor ions based on three-dimensional information of a retention time, a mass-to-charge ratio and an intensity. Whether or not MS3 analysis is performed for each ion is determined beforehand based on whether or not the isotopic distributions of a plurality of ions are superimposed at each retention time rt1 to rt3. Ions (peaks P21 and P22) for which MS3 analysis is performed and ions (peaks P11 and P32) for which MS3 analysis is not performed can be hereby determined at the time the MS spectrum is measured to detect MS/MS precursor ions.