There is disclosed a time-of-flight mass spectrometer capable of cutting out a major portion of carrier gas-derived ions ahead of the ion reservoir. The carrier gas-derived ions are introduced in large quantity from a gas chromatograph and would normally be ionized in large quantity in the ion source 2. The ion source 20 is of the electron impact type and has source magnets 21 for deflecting some of the produced ions away from the center axis of the ion reservoir. Electrostatic lenses 22 for promoting the deflection of the ions caused by the source magnets 21 and a differentially pumped slit 23 for cutting off the deflected ions are mounted downstream of the ion source 20.
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