A charge collection electrode is formed of a plurality of groups each of which is made up of a plurality of adjoining wire electrodes. Further, all the wire electrodes are connected to channels of a signal processing device by the same number of lines as the wire electrodes belonging to one group so that each detection signal outputted from one wire electrode selected from each group is inputted through the same line and so that no two adjoining channels are physically continuous in regard to a certain set of consecutive measurement channels. The signal processing device determines group information indicating to which group the wire electrodes that sent the inputted detection signals belong and outputs a processing signal including the group information to a beam monitor controller. The beam monitor controller determines the position and the beam width of the charged particle beam that passed through the wire electrodes.
