Mass analysis method and mass spectrometer in which the S/N of mass spectra does not deteriorate due to accumulation if an ionization method, such as MALDI, producing spectral intensities that are not uniform in time is employed. Every given number of collected mass spectra are accumulated and stored to produce primary accumulation mass spectra. After the measurements, some of the stored primary accumulation spectra are selected according to a given rule based on a time trace of the intensities of the primary accumulation mass spectra. The selected spectra are accumulated to produce a secondary accumulation mass spectrum.