A quadrupole mass spectrometer includes: a quadrupole mass filter including a pre-rod; an ionization chamber; and an ion optical system that guides ions generated in the ionization chamber to the pre-rod of the quadrupole mass filter, wherein: a potential of an exit side electrode of the ion optical system is lower than a potential of the ionization chamber and a potential of the pre-rod of the quadrupole mass filter; and there is no structure that has a higher potential than the exit side electrode and decelerates the ions between the exit side electrode and the pre-rod.