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, in an in-source CID analysis mode, a DC voltage higher than that applied to a skimmer is applied to a first ion guide so as to create an accelerating electric field in area (B), whereby the ions are sufficiently energized to promote the fragmentation. When the in-source CID is not performed, a DC voltage higher than that applied to the first ion guide is applied to the exit end of a desolvation tube so as to create an accelerating electric field only in area (A) without creating such a field in area (B), whereby both the formation of the cluster ions and the generation of the fragment ions are suppressed, so that a high-quality chromatogram can be obtained.