To reduce space charge phenomenon and sample charging and increase accuracy in measurement of amount of irradiation currents in a gas cluster ion beam processing system.

A vented Faraday cup 302 includes a conductive shock plate 308 with a surface receiving a gas cluster ion beam 128, a vented container surrounding the shock plate 308 and extending in front of it to provide a cup form and including a plurality of coaxial conductive ring electrodes disposed at intervals, and a conducting means conducting currents collected by the shock plate 308 to a current measuring system 358. The conductive ring electrodes are constituted of at least three groups each consisting of at least two ring electrodes and are electrically connected with one another, and the respective groups are biased independently of one another to prevent disadvantageous leakage of charged particles to the inside and the outside of the cup.