An ion implanter performs ion implantation by irradiating a wafer having a notch at its outer peripheral region by an ion beam. In ion implanter, a twist angle adjustment mechanism is configured to adjust a twist angle, an aligner is configured to adjust an alignment angle, a wafer transfer device is configured to transfer the wafer between the aligner and the twist angle adjustment mechanism, an image processing device is configured to detect the twist angle of the wafer on the twist angle adjustment mechanism, and a control device is configured to carry out a twist control in which the wafer is rotated by the twist angle adjustment mechanism by an angle obtained from a first difference between the detected twist angle and the alignment angle and a second difference between the alignment angle and a target twist angle given as one of ion implantation conditions.