1. A mass analysis data analyzing method for obtaining a mass of a target compound by analyzing data of a mass spectrum obtained by a mass analysis on which peaks of a multivalent ion appear, comprising: obtaining a mass spectrum by a mass analysis apparatus; detecting isotopic clusters on the mass spectrum and for deducing a valence of each of the isotopic clusters; obtaining an m/z value which represents each of the detected isotopic clusters; obtaining candidates for an m/z value of a component which has been added to the target compound or desorbed from the target compound in an ionization process, based on a combination of representative points and valences of two or more isotopic clusters which are deduced to originate from a same target compound; evaluating, for the plurality of candidates obtained from different combinations of the plurality of isotopic clusters, a validity of a combination of the candidate m/z values or the isotopic clusters which were a basis of a calculation of the m/z values to finally select one candidate; and deducing the mass of the target compound based on the m/z value and the valence of the selected added/desorbed component.