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 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 revolving about the rotation axis of said rotating gantry with rotation of said rotating gantry for detecting the X-ray emitted from said X-ray generator and outputting X-ray detection signals; and an image information forming unit for forming three-dimensional image information of said irradiation target by using information obtained based on the X-ray detection signals from said plurality of X-ray detectors obtained upon rotation of said rotating gantry, position information of said X-ray generator emitting the X-ray and position information of said plurality of X-ray detector outputting the X-ray detection signals.