An electrostatic lens (3), in which, as an ion incident optical system for sending ions into an orthogonal acceleration unit, five cylindrical electrodes (31-35) are arranged along an ion optical axis (C), and an aperture plate (38) is disposed on a common focal plane of two virtual convex lenses (L1, L2) formed under an afocal condition, is used. The diameter of an aperture opening (39) formed in the aperture plate (38) determines the angular spread of the exiting ion beam. When an applied voltage is set such that the electrostatic lens (3) becomes an afocal system, measurement with a high mass resolution can be performed though the sensitivity is sacrificed slightly, and when an applied voltage is set such that the electrostatic lens (3) becomes a non-afocal system in which an ion passing rate is maximized, measurement with a high sensitivity can be performed though the resolution is sacrificed slightly. Thus, in an orthogonal acceleration-type TOFMS, a mass-resolution priority mode and a measurement sensitivity priority mode can be switched readily.