An ion detector (4) includes a shield electrode (42) between an aperture plate (41) and a conversion dynode (43). The shield electrode (42) has a rectilinearly-moving particle block wall (42a) positioned on an extension line (C′) extending from the central axis (C) of a quadrupole mass filter (3), and an ion attracting electric field adjustment wall (42b) inclined by a predetermined angle θ (acute angle) with respect to the extension line (C′). In the ion attracting electric field adjustment wall (42b) is provided an ion passing aperture (42c). The rectilinearly-moving particles, such as neutral particles, which are ejected from the quadrupole mass filter (3), are blocked by the rectilinearly-moving particle block wall (42a), thereby reducing noises caused by the rectilinearly-moving particles. Meanwhile, the potential of the ion attracting electric field adjustment wall (42b) corresponds to equipotential surfaces in a strong electric field formed by the conversion dynode (43), and thus the condition of the strong electric field is not remarkably changed from the state where no shield electrode (42) is provided. Therefore, the effect of drawing ions is exhibited, thereby maintaining the high ion-detection efficiency.