PURPOSE: To obtain an ion implanter capable of detecting that an ion beam is not scanned in parallel by extracting the applied scanning voltages from scanning electrodes constituting the first and second scanning electrodes respectively, and comparing the scanning voltages in two comparing circuits.

CONSTITUTION: Scanning voltages are extracted from one scanning electrode 41 in the first scanning electrode 4 and one scanning electrode 62 in the second scanning electrode 6 respectively, both scanning voltages are compared in the first comparing circuit 16, if the difference between both voltages becomes a preset value or above, an abnormal signal S is outputted. Scanning voltages are extracted from the other scanning electrode 42 in the first scanning electrode 4 and the other scanning electrode 61 in the second scanning electrode 6 respectively, both scanning voltages are compared in the second comparing circuit 18, if the difference between both scanning voltages becomes a preset value or above, an abnormality signal S is outputted from there. When the first and second comparing circuits 16, 18 are provided, it can simply be detected that an ion beam is not scanned in parallel.