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