A mass spectrometer detector having a coupling capacitor including an anode that is placed at a high potential of say −4.9 kV. An annular electrode is placed outside the anode. A voltage of say −5 kV is applied to the annular electrode. This reduces the potential difference between the anode and the annular electrode, mitigating the electric field. A peripheral electrode is mounted on the outer fringe of a dielectric body and stably held at a potential of say −2.5 kV, for example, by voltage-dividing resistors.