A particle beam therapy apparatus 10 includes: a particle beam irradiator 16 outputting a particle beam B； a movable supporting structure 21 supporting the particle beam irradiator 16； movable plates 37 disposed on a displacement trajectory of the particle beam irradiator 16, forming a substantially horizontal enveloping surface below a table 18 for placing an irradiation object 13, and including first and second floor members in at least one of the movable plates 37, the second floor member being larger in X-ray transmittance than the first floor member； an X-ray generator 27a provided in a non-collision area 31 where the X-ray generator 27a does not collide with any of the particle beam irradiator 16, the supporting structure 21, and the movable plates 37； and an X-ray detector 27b installed at a position where the X-ray detector 27b faces the X-ray generator 27a.
