1. A method of analyzing mass analysis data for deducing a structure of an unknown substance, based on a result of an MS n analysis in which a precursor ion originating from the unknown substance is dissociated into fragment ions by (n−1) steps (where n≧2) and then the fragment ions are subjected to a mass-analyzing process, the method comprising steps of: a) comparing a mass spectrum of the unknown substance obtained by the MS n analysis and a mass spectrum of a known substance similar to the unknown substance and then detecting a common fragment ion peak existing in both of the mass spectra; b) identifying a partial structure of the known substance, the partial structure corresponding to the common fragment ion peak; c) determining the partial structure as a partial structure of the unknown substance; d) calculating a difference value between a mass-to-charge ratio of a fragment ion and a mass-to-charge ratio of a precursor ion at each dissociating step of an MS n analysis of the unknown substance, and/or a difference value between a mass-to-charge ratio of a fragment ion at each dissociating step of the MS n analysis and a mass-to-charge ratio of a fragment ion or a precursor ion at the previous step, which were original ions of the fragment ion; e) comparing the difference value between the mass-to-charge ratios regarding the unknown substance and the difference value preliminarily calculated between the mass-to-charge ratios regarding the known substance, and then detecting a common difference value; f) identifying a partial structure of the known substance, the partial structure corresponding to the common difference value between the mass-to-charge ratios detected in step e); and g) determining the partial structure as a partial structure of the unknown substance.