An orthogonal acceleration time-of-flight mass spectrometer (1) includes: an ion ejector (123) which ejects measurement-target ions in a predetermined direction; an orthogonal accelerator (132) which accelerates ions in a direction orthogonal to the direction in which the ions are ejected; a ring electrode (131) located between the ion ejector and the orthogonal accelerator, the ring electrode having an opening for allowing ions to pass through and arranged so that the central axis (C2) of the opening is shifted from the central axis (C1) of the ion ejector in a direction along the axis of the acceleration of the ions by the orthogonal accelerator; a reflectron electrode (134) which creates a repelling electric field for reversing the direction of the ions accelerated by the orthogonal accelerator; and an ion detector (135) which detects ions after the direction of flight of the ions is reversed by the reflectron electrode.