The present disclosure provides an ion detector for improving the effect of electric field for pulling in an ion to be detected to a first-stage electrode of a secondary electron multiplier (SEM), and improving the effect of a stray light reduction. In one example embodiment, an ion detector includes a SEM, and a lead-in electrode for pulling in an ion to a first-stage electrode side of the SEM. At least one of the area of the lead-in electrode and a potential difference between the lead-in electrode and neighboring electrodes of the lead-in electrode, the neighboring electrode being an electrode not of the SEM, is set so that the light amount of internal-stray light generated inside the detector entering the first-stage electrode is not more than that of external-stray light generated outside the detector entering the first-stage electrode, when an ion is introduced into the detector.