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, the method comprising the following steps: a) an isotopic cluster detection step and a valence deduction step, wherein, in the isotopic cluster detection step, isotopic clusters appearing on the mass spectrum are detected and, in the valence deduction step, a corresponding valence of each of the isotopic clusters detected in the isotopic cluster detection step is deduced; b) a representative point determination step for determining a representative point for each detected isotopic cluster by obtaining, for each of the detected isotopic clusters, a corresponding m/z value being said representative point, which represents a detected isotopic cluster; c) a candidate extraction step for obtaining a plurality of 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 different combinations of representative points and valences of the detected isotopic clusters which detected isotopic clusters are regarded to originate from a same target compound, said plurality of candidates for m/z values of said component are obtained by extracting, using a range of m/z values of components which range has been determined in advance and which components are estimated to be added to a same target compound or to be desorbed from a same target compound in the ionization process, out of the detected isotopic clusters, at least two isotopic clusters of different valences which are deduced to originate from the same component, and calculating of-m/z values of said plurality of candidates of said component based on said detected representative points and valences of the detected isotopic clusters corresponding to the extracted isotopic clusters; d) an added/desorbed component selection step (S6-S13) for evaluating, for the plurality of candidates obtained from different combinations of the plurality of the detected isotopic clusters in the candidate extraction step, a validity of candidate m/z values or of a combination of the isotopic clusters which were a basis of a calculation of the m/z values of components to refine the candidates for added/desorbed components to finally select one candidate; and e) a compound deduction step for deducing the mass of the target compound based on the m/z value and the valence of the selected added/desorbed component. a) an isotopic cluster detection step and a valence deduction step, wherein, in the isotopic cluster detection step, isotopic clusters appearing on the mass spectrum are detected and, in the valence deduction step, a corresponding valence of each of the isotopic clusters detected in the isotopic cluster detection step is deduced; b) a representative point determination step for determining a representative point for each detected isotopic cluster by obtaining, for each of the detected isotopic clusters, a corresponding m/z value being said representative point, which represents a detected isotopic cluster; c) a candidate extraction step for obtaining a plurality of 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 different combinations of representative points and valences of the detected isotopic clusters which detected isotopic clusters are regarded to originate from a same target compound, said plurality of candidates for m/z values of said component are obtained by extracting, using a range of m/z values of components which range has been determined in advance and which components are estimated to be added to a same target compound or to be desorbed from a same target compound in the ionization process, out of the detected isotopic clusters, at least two isotopic clusters of different valences which are deduced to originate from the same component, and calculating of-m/z values of said plurality of candidates of said component based on said detected representative points and valences of the detected isotopic clusters corresponding to the extracted isotopic clusters; d) an added/desorbed component selection step (S6-S13) for evaluating, for the plurality of candidates obtained from different combinations of the plurality of the detected isotopic clusters in the candidate extraction step, a validity of candidate m/z values or of a combination of the isotopic clusters which were a basis of a calculation of the m/z values of components to refine the candidates for added/desorbed components to finally select one candidate; and e) a compound deduction step for deducing the mass of the target compound based on the m/z value and the valence of the selected added/desorbed component.