Provided is a mass spectrometer including: a prediction equation storage section 34 configured to store a plurality of prediction equations which respectively correspond to different kinds of isotopic ions originating from a compound, each prediction equation expressing a relationship between m/z of a monoisotopic ion of the compound and a ratio between the signal intensity at the monoisotopic ion of the compound and the signal intensity of an isotopic ion in a mass spectrum; a measurement section 1 configured to perform chromatograph mass spectrometry on a sample to collect data; a predicted mass spectrum creation section 33 configured to calculate a predicted value of the signal intensity ratio related to each isotopic ion, by applying, in the prediction equations, m/z information corresponding to a peak observed on a measured mass spectrum and information of a given charge state of the ion, and to create a predicted mass spectrum based on the predicted values; and a mass spectrum evaluation section 33 configured to evaluate the possibility that a plurality of peaks in the measured mass spectrum are peaks originating from one compound, by comparing the predicted mass spectrum and the measured mass spectrum.