A beam current adjuster for an ion implanter includes a variable aperture device which is disposed at an ion beam focus point or a vicinity thereof. The variable aperture device is configured to adjust an ion beam width in a direction perpendicular to an ion beam focusing direction at the focus point in order to control an implanting beam current. The variable aperture device may be disposed immediately downstream of a mass analysis slit. The beam current adjuster may be provided with a high energy ion implanter including a high energy multistage linear acceleration unit.