an ion implanter having means for scanning an ion beam on a wafer is provided , wherein the scanning means , on which the wafer is mounted , moves the wafer in a region where the ion beam is irradiated . a detecting means , which is fixedly mounted adjacent to the scanning means , detects the ion beam that is overly scanned out of the scanning means . the detecting means has an inclined surface so that a portion of the detecting means adjacent to the scanning means is positioned below a surface of the wafer that is disposed on the scanning means . accordingly , the ion implanter may prevent the wafer in the scanning means from being polluted with sputtering particles generated from a surface of the scanning means .