In a first-stage intermediate vacuum chamber, cluster ions causing a background noise are dominantly formed in area (A), while fragment ions are dominantly generated in area (B). Taking this fact into account, when no in-source CID analysis is performed, voltages applied to the first-stage plate electrode of an ion guide and the exit end of a desolvation tube are adjusted to create an accelerating electric field only in area (A) without creating such a field in area (B). Meanwhile, voltages applied to the electrodes of the ion guide are adjusted to create an electric field for separating ions according to their mobility and selecting a specific ion. Such an operation suppresses the cluster-ion formation, removes ions which originate from impurities and have mass-to-charge ratios close to or equal to those of the ions originating from a target substance, and suppresses the fragment-ion generation. As a result, the target ions are detected with high S/N.