A mass spectrometer, including an evacuable vessle, mass separation means provided in the evacuable vessel for separating ions in accordance with the mass thereof, and ion detection means provided in the evacuable vessel for detecting ions emitted from the mass separation means to convert the emitted ions into an electric signal, in which the ion detection means includes an electron-multiplier for detecting positive ions and a photo-multiplier for detecting negative ions. According to this mass spectrometer, positive ions can be detected at high sensitivity, and negative ions are readily detected.