An ion trap time-of-flight mass spectrometer capable of obtaining highly-sensitive mass spectra even on the lower mass number side is realized. The ion trap time-of-flight mass spectrometer includes an ion source that operates at atmospheric pressure, an ion optical system for introducing the ions generated by the ion source into a vacuum chamber and converging the ions introduced into the vacuum chamber, an ion trap part for trapping ions in the vacuum chamber, a multipole part for converging the kinetic energy of the ions discharged from the ion trap, and a time-of-flight mass spectrometry part for measuring the ions discharged from the multipole part. The period of high-voltage pulses generated by an electrode provided in the time-of-flight mass spectrometry part can be changed depending on an ion content introduced into the multipole part.