Provided is a mass spectrometer including: a measurement condition setter (42) configured to set a plurality of measurement conditions which are different from each other in terms of the set value of at least one measurement parameter; a measurement executer (43) configured to acquire a plurality of sets of mass spectrometric data respectively corresponding to the plurality of measurement conditions; a product ion extractor (44) configured to extract product ions detected with intensities exceeding a previously determined reference value; an MRM spectrum element information creator (45) configured to determine the mass-to-charge ratios and measured intensities of the extracted product ions, the mass-to-charge ratio of the precursor ion, as well as the measurement condition, and to create a plurality of pieces of MRM spectrum element information; an MRM spectrum composer (46) configured to compose an MRM spectrum from the mass-to-charge ratios and the measured intensities of the product ions included in the plurality of pieces of MRM spectrum element information; and a library data creator (47) configured to relate the MRM spectrum to information concerning the target compound to create library data for the target compound.