PURPOSE: To protect an irradiated target from an abnormal ion beam current by providing a detection system for monitoring ion beam current, and interrupting the irradiation when a fixed value or more of ion beam is emitted to the irradiated target.

CONSTITUTION: When an ion beam 11a is emitted to an irradiated target 15 from an ion generating source 11, the ion beam 11a is deflected and driven by a beam deflecting and driving means 12, and the ion beam 11a emitted to the irradiated target 15 is detected by a beam detecting means 13 and outputted to a control means 14 as a beam detection signal Si. In the control means 14, the beam detection signal Si is compared with a preset beam standard signal SL, and when the beam detection signal Si exceeding the beam standard signal SL is detected, drive control is conducted so that the ion beam 11a is not emitted to the irradiated target 15. Thus, the irradiated target 15 can be protected from the abnormal ion beam 11a.