An object of the present invention is to provide a mass spectrometer that uses a time-of-flight mass spectrometer for performing mass spectrometry on the basis of the difference in flight time based on mass of desired ions, and that is suitable for improving the sensitivity and analysis precision of the mass spectrometer. A gate electrode is located at a stage before an acceleration region that is located before an emitter for emitting ions. This gate electrode is capable of applying the voltage that is set on a mass-number region basis, and is also capable of separating desired ions to be measured on the basis of the mass number by switching the gate electrode at high speed. Therefore, it is possible to improve the resolution.