A radiation therapy system comprising: a therapeutic radiation irradiation apparatus configured to irradiate a therapeutic beam to a target in a subject; at least two fluoroscopic radiation photographing apparatuses (4A, 4B) configured to acquire fluoroscopic images of a tracking object simultaneously from at least two directions by fluoroscopic radiation; a tracking object position computation apparatus configured to compute a three-dimensional position of the tracking object from fluoroscopic images acquired by the fluoroscopic radiation photographing apparatuses (4A, 4B); a therapeutic beam irradiation control apparatus configured to judge whether or not the target exists in a predetermined irradiation region on the basis of the three-dimensional position of the tracking object computed by the tracking object position computation apparatus and control the therapeutic beam irradiation apparatus so as to irradiate the target with the therapeutic beam when the target is judged to exist in the irradiation region characterised by further comprising: a fluoroscopic radiation photographing control apparatus configured to control irradiation dose per unit time of the fluoroscopic radiation photographing apparatuses (4A, 4B) on the basis of the three-dimensional position of the tracking object, wherein the fluoroscopic radiation photographing control apparatus is configured to determine whether or not the three-dimensional position of the tracking object is included in a standard photographing region which is a region obtained by expanding or contracting the irradiation region by a predetermined amount, and to control the irradiation dose per unit time of the fluoroscopic radiation photographing apparatuses based on the determination result. a therapeutic radiation irradiation apparatus configured to irradiate a therapeutic beam to a target in a subject; at least two fluoroscopic radiation photographing apparatuses (4A, 4B) configured to acquire fluoroscopic images of a tracking object simultaneously from at least two directions by fluoroscopic radiation; a tracking object position computation apparatus configured to compute a three-dimensional position of the tracking object from fluoroscopic images acquired by the fluoroscopic radiation photographing apparatuses (4A, 4B); a therapeutic beam irradiation control apparatus configured to judge whether or not the target exists in a predetermined irradiation region on the basis of the three-dimensional position of the tracking object computed by the tracking object position computation apparatus and control the therapeutic beam irradiation apparatus so as to irradiate the target with the therapeutic beam when the target is judged to exist in the irradiation region characterised by further comprising: a fluoroscopic radiation photographing control apparatus configured to control irradiation dose per unit time of the fluoroscopic radiation photographing apparatuses (4A, 4B) on the basis of the three-dimensional position of the tracking object, wherein the fluoroscopic radiation photographing control apparatus is configured to determine whether or not the three-dimensional position of the tracking object is included in a standard photographing region which is a region obtained by expanding or contracting the irradiation region by a predetermined amount, and to control the irradiation dose per unit time of the fluoroscopic radiation photographing apparatuses based on the determination result.