An off-axis ion transport optical system (20) including a front-stage quadrupole ion guide (21), a rear-stage quadrupole ion guide (22), and an ion deflector (23) is disposed inside an intermediate vacuum chamber (2) in a stage next to an ionization chamber (1) maintained at an atmospheric pressure. Both of the quadrupole ion guides (21 and 22) have the same configuration as that of a conventional ion guide that transports ions while trapping the ions using a radio-frequency electric field. The ion deflector (23) includes a pair of parallel flat electrodes (231 and 232) and deflects ions using a direct-current electric field. By causing the deflected ions to reach the ion receiving range of the rear-stage quadrupole ion guide (22), it is possible to efficiency introduce ions while deflecting the ions. Meanwhile, the ions and neutral particles are separated from each other in the ion deflector (23). This provides an off-axis structure ion transport optical system that achieves a high ion transmission efficiency with a simple structure.