There is provided a mass spectrometry data processor allowing for easy selection of a compositional formula for a sample component to be analyzed (analyte) even in general mass spectrometry applications. The mass spectrometry data processor is used to perform a qualitative analysis of the sample component based both on a first mass spectrum obtained by ionizing the sample component by a soft ionization method and on a second mass spectrum generated by cleavage of the sample component. The data processor includes: a molecular ion peak detector for detecting one molecular ion peak from the first mass spectrum; a fragment ion peak detector for detecting plural fragment ion peaks from the second mass spectrum; a composition estimator for obtaining estimated compositional formulas for the sample component from the molecular ion peak and obtaining estimated compositional formulas for fragments constituting the sample component from the fragment ion peaks; an assignment validity decision device for making decisions as to whether the estimated compositional formulas for the fragments can be assigned to the estimated compositional formulas for the sample component; and a degree of coincidence computing section for computing degrees of coincidence of the estimated compositional formulas for the fragments with the estimated compositional formulas for the analyte based on results of the decisions made by the assignment validity decision device.