A method for selecting product ions at the time of performing multiple reaction monitoring (MRM) using an MS/MS type mass spectrometer is provided. Plurality of product ion scan events is prepared for certain precursor ions. In the product ion scan events, parameters for determining the cleavage conditions of the precursor ions are changed. Next, the precursor ions are introduced into a collision cell and the plurality of product ion scan events is executed, thereby obtaining a plurality of mass spectra corresponding to the respective product ion scan events. Then, the plurality of mass spectra is compared with one another, and ions corresponding to the mass peak with the highest appearance frequency are selected as product ions. Alternatively, ions having the highest cumulative signal intensity obtained by integrating the plurality of mass spectra are selected as product ions. Accordingly, optimum product ions for certain precursor ions are selected.