Disclosed is a mass spectrometer, which includes an ion optical system for converging ions and transporting the ions to a subsequent stage. Instead of a conventional rod-shaped electrode, the ion optical system includes an electrode formed of a thin metal plate member. Specifically, each of four electrodes 41a to 41d formed of the metal plate members has an edge surface facing an ion optical axis C and extends along the ion optical axis C. Further, the electrodes 41a to 41d are disposed around the ion optical axis C in a radial pattern while keeping an angle of 90.degree. between the adjacent electrodes. The present invention can provide the ion optical system at a lower cost.