A single type quadrupole mass spectrometer equipped with an ion source by the ESI method, which is a small device including a vacuum pump having a relatively small evacuation speed. The internal diameter of a desolvation tube for introducing ions from an ionization chamber into a first intermediate vacuum chamber is set to 0.4 mm φ, which is large for a small mass spectrometer. The evacuation speed of a rotary pump is determined so that the product of the cross-sectional opening area of the desolvation tube and the pressure in the first intermediate vacuum chamber falls within a range of 15 to 40 mm2·Pa. This can ensure high detection sensitivity and reduce clogging of the desolvation tube due to droplets. Since the pressure in the first intermediate vacuum chamber does not need to be increased more than necessary, a small rotary pump having a small evacuation speed can be used.