Disclosed is an ion trap mass spectrometer for MSn analysis, which comprises a frequency-driven ion trap section operable to trap sample ions and isolate a precursor ion from the sample ions, while setting an ion-trapping RF voltage waveform at a first frequency providing a first low-mass cutoff (LMCO) value, and, then after setting the ion-trapping RF voltage waveform at a second frequency greater than the first frequency to provide a second LMCO value less than the first LMCO value, without changing an amplitude of the ion-trapping RF voltage waveform, to irradiate the precursor ion in a trapped state with light so as to photodissociate the precursor ion into fragment ions; and an analyzer section operable to subject the fragment ions ejected from the ion trap section, to mass spectrometry so as to obtain information about a molecular structure of the precursor ion. The ion trap mass spectrometer of the present invention can maximize a mass range coverable in one cycle of MSn analysis.