A mass spectrometer for analyzing ultra trace element using plasma ion source comprising, a plasma generating means for ionizing sampling gas by generating plasma, a vaccum chamber for taking in ions of the sampling gas from a hole of the vacuum chamber, an ion lens and a mass analyzing portion, and an ion detector for detecting the ions which are passed through the ion lens and the mass analyzing portion, wherein further comprising, a moving mechanism for moving said plasma generating means according to a vacuum degree measured in the vacuum chamber so as to make the sensitivity of the mass spectrometer higher.