A particle beam therapy device 10 is provided with: a particle beam irradiator 16 for emitting a particle beam B； a movable support body 21 for supporting the particle beam irradiator 16； a plurality of moving boards 37 that are provided on a trajectory of displacement of the particle beam irradiator 16 and are configured so as to form a substantially horizontal envelope surface below a tabletop 18 on which a body to be irradiated 13 is placed, at least one of the moving boards being provided with a first floor member and a second floor member having a greater X-ray transmittance than that of the first floor member； X-ray generators 27a provided to a non-collision area 31 in which the X-ray generators do not collide with the particle beam irradiator 16, the support body 21, or any of the moving boards 37； and X-ray detectors 27b that are installed at positions facing the X-ray generators 27a and that detect X-rays transmitted through the first floor member.
