A mass spectrometer is provided in which ions are favorably introduced into a loop orbit or favorably led out from the loop orbit without affecting the motion of the ions flying along the loop orbit. An ion-introduction orbit 5 is set to correspond to the orbit (ejection orbit portion 4) of ions after being bent by the sector-shaped electric field E1 in the loop orbit 4. When ions are introduced, a voltage applied to the electrode unit 11 is put to zero to release the sector-shaped electric field E1. Then the ions emitted along the ion-introduction orbit 5 fly straight in the electrode unit 11. The direction and position of the ions coming out from the exit end of the electric field is the same as those ions flying along the loop orbit 4. Therefore, there is no need for placing a deflection electrode for introducing/leading-out ions on the loop orbit.