Provide is an ion trap mass spectrometer which is configured to gain an MS spectrum of only fragment data in an MS/MS analysis, thereby makes it possible to perform the analysis in a short period. For this purpose, the device is comprised of: an ionization unit configured to ionize a sample which has been separated into respective components; an ion trap unit configured to trap ions ionized by ionization unit in an electric field and eject the ions in accordance with the respective masses of the ions; a detection unit configured to detect the ions ejected from the ion trap unit; and a processing unit configured to generate an MS spectrum (mass spectrum) on the basis of data detected in the detection unit. The processing unit further configured to gain an MS spectrum of only fragment data of a target ion from a difference between an MS spectrum gained in an MS analysis made before and/or after an MS/MS analysis and an MS spectrum gained in the MS/MS analysis.