A mass spectrometer includes an ionization chamber (100) which generates fragment-free ions to be detected from an introduced gas to be detected, and a mass spectrometer chamber (140) including a mass spectrometer (160) which fractionates by mass the ions to be detected that are transported from the ionization chamber and which detects the ions. The mass spectrometer further includes a probe (111) which holds a liquid sample or a solid sample and causes the liquid sample or the solid sample to generate the gas to be detected upon heating by a heating means, and a gas introduction means (170) which introduces a predetermined gas from the probe to the ionization chamber to transport, to the ionization chamber, the gas to be detected that is generated at the probe.