A mass spectrometry unit of the present invention includes a mass spectrometry portion that detects ion current values of a gas to be measured according to mass-to-charge ratio, to thereby measure partial pressures of the gas to be measured. The mass spectrometry unit further includes: a control portion for preliminary storing a record of a mass-to-charge ratio of a specific gas that decreases a function of a specific portion of the mass spectrometry unit, in which if an ion current value with the mass-to-charge ratio of the specific gas detected by the mass spectrometry portion is not less than a predetermined value, the control portion outputs a warning signal denoting a functional decrease in the specific portion.