A mass spectrometer includes first to fifth vacuum chambers and a single split-flow turbo molecular pump. The first vacuum chamber draws an ionized sample from atmospheric pressure plasma. The second vacuum chamber includes means for extracting and guiding an ion beam from ions from the first vacuum chamber. The third vacuum chamber includes first ion-optical separator means. The fourth vacuum includes a collision/reaction cell into which collision/reaction gas is introduced. The fifth vacuum chamber includes second ion-optical separator means and a detector. The split-flow turbo molecular pump has multiple inlets for differentially evacuating the second, third, fourth and fifth vacuum chambers. The fourth vacuum chamber is connected to an inlet of the split-flow turbo molecular pump providing a pressure higher than a pressure of an inlet of the split-flow turbo molecular pump to which the third vacuum chamber is connected.