An ion implanter is provided with a system for monitoring parameters of the ion implanter in real time to control respective components in the ion implanter. This system is allowed to have a function of calculating an accumulated dose distribution during ion implantation treatment and correcting a mechanical scan speed of a wafer holding section in a Y direction so as to render an accumulated dose uniform, a function of changing a magnetic field of a mass analyzing section to thereby control a center position of an ion beam, and a function of varying a suppression voltage of an aperture and an ion beam current to control a diameter of the ion beam.