To enable more reliable detection of ion peaks from mass spectral data. Ion peaks are detected from mass spectrum by the following steps. A step of acquiring mass spectral data made up of peaks which is acquired using a mass spectrometer, the peaks having mass-to-charge ratio and intensity information, a step of classifying the peaks in the acquired mass spectral data into a plurality of classes according to the intensity of the peaks, and a step of identifying the peaks as ion peaks or noise peaks based on the intensity information of the peaks which have been classified into the plurality of classes.