In an imaging mass spectrometer for analyzing the same kind of samples using results of imaging mass spectrometric analysis performed on those samples, a measurement section (1) acquires mass spectrometric data by performing an analysis on each of the micro areas on a sample. A region-of-interest setter (32) sets an ROI on each sample, and divides each ROI into the same number of subregions each including the micro areas so that the subregions correspond to each other on the samples respectively covering roughly identical sites on the samples. An individual-index-value calculator (33) calculates an individual index value for each subregion, using mass spectrometric data acquired at the micro areas in the subregion, the individual index value reflecting a similarity or difference among the samples in terms of a degree of expression of each m/z value. A general-index-value calculator (34) calculates a general index value for each m/z value among the ROIs of the samples, using the individual index values calculated for the ink values for each subregion included in each ROI.