1. A mass spectrometry data processing method for identifying a substance contained in each of a plurality of fractionated samples obtained by separating various substances contained in a sample according to a predetermined separation parameter and fractionating the sample, based on MS n spectra obtained by MS n analyses (where n is an integer equal to or greater than two) respectively performed for the plurality of fractionated samples, comprising: a) an identification probability estimation model creation step, in which: (a1) order information for determining an order of MS n-1 peaks found by MS n-1 analyses for a plurality of fractionated samples obtained from a preparatory sample is derived from information on the MS n-1 peaks and a result of substance identification based on results of MS n analyses which respectively use the MS n-1 peaks as a precursor ion, (a2) an identification probability estimation model is created on a basis of a relationship between a cumulative number of MS n peaks and a number of successful identifications determined through a series of MS n analyses and identifications in which a plurality of MS n-1 peaks originating from a same kind of sample are sequentially selected as a precursor ion according to the aforementioned order, and (a3) the aforementioned order information, and identification probability estimation model information representing the aforementioned identification probability estimation model, are memorized; b) a peak order calculation step, in which, for MS n-1 peaks found by an MS n-1 analysis of at least one fractionated sample obtained from a sample to be identified, the order of the MS n-1 peaks is calculated by using the order information; and c) an identification probability estimation step, in which an estimated value of the identification probability of each of the MS n-1 peak is calculated from the order of the MS n-1 peaks calculated in the peak order calculation step, with reference to the identification probability estimation model derived from the identification probability estimation model information, wherein the estimated value of the identification probability for an MS n analysis and identification using, as a precursor ion, an MS n-1 peak corresponding to a fractionated sample obtained from the sample to be identified is obtained before the MS n analysis is performed.