In a mass spectrometer, an ion of a sample gas is produced in an ion source by colliding the gas with an electron beam, and is introduced into a magnetic field or an electric field. The ion is separated based on the mass number, and the sample is analyzed from a mass spectrum. The ion source has an electrode made of stainless steel, which is baked at a temperature in a range from 200.degree. C. to 700.degree. C. in an air atmosphere.
