1. A mass-analysis data processing method for processing mass-analysis data collected by a mass spectrometer, comprising: performing a mass analysis on each of a plurality of micro areas within a two-dimensional area on a sample and acquiring an optical microscope image on the sample; a small-area specifying step, in which, based on a visual judgment on the optical microscope image taken for a predetermined area on the sample, a small area that can be regarded as having a same composition or exhibiting a same property is specified for each of two or more portions having different compositions or exhibiting different properties; an expression information extracting step, in which, for each small area specified in the small-area specifying step as an area having the same composition or exhibiting the same property, the mass-analysis data obtained for all the micro areas included in the small area are processed to extract, as expression information of the small area, peak information that is highly common among the micro areas; and a specific expression information extracting step, in which the small areas having different compositions or exhibiting different properties are compared in terms of their expression information to extract, for each small area, specific expression information from all the expression information of the small area.