In conducting multiple repetitions of MS/MS analysis on the same test sample for which a precursor ion whose m/z is known (m/z=M) has been established, MS/MS analysis is conducted under a dissociation condition in which CID is less prone to occur in part of the analysis. When an MS/MS spectrum is created by summing up spectral data thus obtained, a known precursor ion is observed at m/z=M without exception. Thus, a peak corresponding to the precursor ion is detected on the MS/MS spectrum, a mass deviation between an actual measured value and theoretical value M of m/z at the peak is determined, and a spectrum is created by correcting other peaks for mass shifts based on the mass deviation. This makes it possible to mass-calibrate the MS/MS spectrum in substantially the same manner as an internal standard method and improve mass accuracy over conventional methods.