Peak information is collected from an MS/MS spectrum in which the product ions generated from ions originating from a plurality of compounds are mixed. Using one of the m/z values of a plurality of ion species as precursor-ion information and the peak information of the MS/MS spectrum as product-ion information, a database search is performed to find candidate compounds for each compound, and a standard MS/MS spectrum of each candidate compound is obtained. For each combination of the candidate compounds, a virtual MS/MS spectrum is created by integrating the standard MS/MS spectra of those candidate compounds, and the degree of similarity between the virtual MS/MS spectrum and the measured MS/MS spectrum is calculated. If the candidate compounds are correct compounds, the degree of similarity will be high. Accordingly, the compound names and other results are visually presented to users in descending order of the degree of similarity.