Based on the mass spectrum obtained by mass-analyzing a sample, the composition of the unknown substance is deduced, and after that, an MS/MS analysis is performed in which the unknown substance is set to be a precursor ion. Then, based on the peaks appearing on the MS/MS spectrum, the actually measured mass of each product ion is obtained (S1 through S4). On the other hand, the compositions of the product ion generated by the dissociation of the unknown substance are obtained by the combination, i.e. the condition, of the kind of the constituent element and the number of each element of the unknown substance's deduced component. Then, it is checked whether or not the theoretical mass in consistency with the actually measured mass of the product ion exists (S5). In the case where one which is consistent with a theoretical mass is not existent, it is possible to determine that the original deduction of the known substance's composition has been incorrect. Therefore, this result is given to the composition deduction as feedback to refine the candidates for the unknown substance's composition (S6).