An electrostatic lens (3), including five cylindrical electrodes (31-35) arrayed along an ion-optical axis (C) and an aperture plate (38) located on a common focal plane of two virtual convex lenses (L1 and L2) formed under an afocal condition, is used as an ion-injecting optical system for sending ions into an orthogonal acceleration unit. The diameter of a restriction aperture (39) formed in the aperture plate (38) determines the angular spread of an exit ion beam. When voltages for making the electrostatic lens (3) function as an afocal system are set, a measurement with high mass-resolving power can be performed at a slight sacrifice of the sensitivity. When voltages for making the lens function as a non-afocal system having the highest ion-passage efficiency are set, a measurement with high sensitivity can be performed at a slight sacrifice of the resolving power.