A beam data processing apparatus has a plurality of channel data conversion units that convert a plurality of analogue signals outputted from a position monitor into digital signals, a position size processing unit that calculates a beam position, based on voltage information items obtained through processing by the plurality of channel data conversion units, an abnormality determination processing unit that determines the beam position and generates a position abnormality signal, and an integrated control unit that controls the plurality of channel data conversion units in such a way that while a beam is stopped at an irradiation spot, digital signal conversion processing is implemented two or more times; the channel data conversion unit has a plurality of A/D converters, a demultiplexer that distributes analogue signals, and a multiplexer that switches respective digital signals processed by the A/D converters so as to output them to the position size processing unit.