The ion implanter includes lens elements that arrange unit lens elements along a direction of a beam width of a ribbon ion beam and regulate a magnetic field or electric field to be created by each unit lens element in order to regulate a current density distribution of the ion beam, and a controlling portion that sets the intensity of the magnetic field or electric field to be created by the unit lens element to be regulated by the lens elements in accordance with the measured current density distribution. The regulation intensity of the magnetic field or electric field to be created by the unit lens element that corresponds to a position to be regulated in the unit lens elements of the lens elements is determined from the measured current density distribution and a value obtained by multiplying the determined regulation intensity by a fixed ratio is determined as the regulation intensity of the magnetic field or electric field for the magnetic field or electric field to be created by a unit lens element adjacent to the unit lens element.