An ion implanter has a beam deflector having a pair of magnetic poles facing each other in a z direction, insulating members provided on the respective magnetic poles, at least one pair of electrodes provided on the insulating members so as to face each other across a space through which the ion beam passes in the z direction, and at least one power source configured to apply a voltage to the pair of electrodes. The beam deflector is configured to deflect, by a magnetic field, an overall shape of the ion beam so as to be substantially parallel to the x direction. The pair of electrodes have a dimension longer than the dimension of the ion beam in the y direction, and constitute an asymmetrical einzel lens in the direction of travel of the central orbit of the ion beam.