A particle therapy system has an irradiation system attached to a rotary drum of a gantry. A radiation treatment cage disposed in the rotary drum includes a movable floor including a horizontal floor portion. The movable floor includes a number of footboards connected bendably and X-ray transmission plates. The movable floor has a slide member at each end thereof, and the slide member is movably attached to a guide rail that is provided for each of opposite side surfaces of the irradiation system. X-ray sources are disposed outside the rotary drum apart from each other in a circumferential direction of the rotary drum and attached to the outer surface of the rotary drum. The irradiation system includes X-ray detection systems opposite to the X-ray sources.