1. A therapy system using an ion beam, in which an ion beam for treatment is delivered to an irradiation target, said therapy system comprising: a rotating gantry; a beam generator for generating said ion beam; a beam delivery unit mounted to said rotating gantry and delivering said ion beam; an X-ray generator generating an X-ray and mounted to said rotating gantry, said X-ray generator being capable of moving to a position of a beam path of said ion beam, revolving about a rotation axis of said rotating gantry with rotation of said rotating gantry and emitting X-ray at a plurality of rotation angles of said rotating gantry; a plurality of X-ray detectors for detecting the X-ray emitted from said X-ray generator and outputting X-ray detection signals, said plurality of X-ray detectors being mounted to said rotating gantry and revolving about the rotation axis of said rotating gantry with rotation of said rotating gantry; and a tomographic information forming unit for forming tomographic information of said irradiation target by using information obtained based on the X-ray detection signals outputted respectively from said plurality of X-ray detectors.