Abstract: A fixed ring rail and a moving ring rail sandwiching a moving route of a radiation beam irradiating section therebetween, forms a cylindrical passage with a semicircular cross-section having a horizontal floor. Upon that passage is placed a moving floor composed of a continuous series of multiple plates which are connected freely flexible with each other. The moving floor is moved in synchrony with the rotation of radiation beam irradiation section, and, at the same time, the moving ring rail is allowed to rotate in the reverse direction by the same distance. A horizontal floor necessary as an access floor to the treatment bed and to the radiation beam irradiating section is formed regardless of the rotating position of the radiation beam irradiation section, and the patient is surrounded completely to remove him/her of fear.