In a mass analysis of a sample, candidate compositions Y of a fragment ion produced by a dissociating operation are deduced from the mass of that fragment ion (Steps S6 to S9). If the number of the candidates Y is larger than a predetermined value (“No” in Step S10), the repetition counter of the dissociating operation is increased by one and the mass analysis of the fragment ion is performed again. If the number of the candidates is equal to or smaller than the predetermined value, the difference between the masses of the fragment ions before and after each mass-analyzing stage is calculated (Step S11). From this mass difference, the candidates Z of the desorption ion at each stage is deduced (Step S12). These candidates Z and Y are used to narrow down the candidate composition formulae X deduced from the mass of the precursor ion (Step S13). If the number of the candidates has decreased to one or become equal to or smaller than a predetermined value, the result is displayed (Steps S14 and S15). Thus reducing the number of the candidates to the lowest possible value, the present method provides the user with useful information for analyzing the molecular structure and/or composition of a sample having a large molecular weight.