An apparatus is provided. The apparatus includes a beam current measuring device and a first electrode. The beam current measuring device is retractably movable into an ion beam trajectory so as to measure an ion beam current. The first electrode is disposed immediately upstream of the beam current measuring device in an ion beam transport channel. The first electrode serves both as a suppressor electrode for repelling secondary electrons released from the beam current measuring device, back toward the beam current measuring device, and as a beam optical element other than the suppressor electrode.