In radiography the human body must be accurately and quickly positioned under X-ray equipment in order to obtain an image of the examined organ. The good positioning skill of the radiological technologist is essential for obtaining accurate diagnostic information of an examined organ and for reducing X-ray exposure in radiography. In this study, we developed a computed tomography (CT) image-based training system which could assist the students to train the positioning practice. By using a personal computer (PC) the positioning could be simulated. First, from the CT image data of the phantom scanned beforehand, we constructed the three-dimensional (3D) phantom based on a solid model and we performed the positioning of the 3D phantom on the PC. Next, we calculated the plane image from the CT image data corresponding to the positioned phantom and displayed the calculated plane image. By using our proposed training system, it was expected that students could perform the positioning training at the same time without having to expose X-rays and develop the films.