A charged particle beam position monitor is provided with a plurality of position monitors and a beam data processing device that performs calculation processing of the state of a charged particle beam, based on a plurality of signals outputted from the position monitors. The beam data processing device includes a plurality of channel data conversion units that perform AD conversion processing of the plurality of signals outputted from the position monitors; a position size processing unit, for each of the position monitors, that calculates the beam position of the beam, based on voltage information obtained through the AD conversion processing; and an integrated control unit that controls the plurality of channel data conversion units in such a way that while the beam is irradiated onto an irradiation subject, AD conversion processing of the signals is performed at different timings for the respective position monitors.