A beam line before incidence on a beam scanner is arranged with an injector flag Faraday cup that detects a beam current by measuring a total beam amount of an ion beam to be able to be brought in and out thereto and therefrom. When the ion beam is shut off by placing the injector flag Faraday cup on a beam trajectory line, the ion beam impinges on graphite provided at the injector flag Faraday cup. At this occasion, even when the graphite is sputtered by the ion beam, since the injector flag Faraday cup is arranged on an upstream side of the beam scanner and the ion beam is shut off by the injector flag Faraday cup, particles of the sputtered graphite do not adhere to a peripheral member of the injector flag Faraday cup.