1. A bed positioning system for a radiation therapy system capable of capturing X-ray transparent images and CT images by rotating an X-ray tube and an X-ray detector around a subject on a bed, comprising: a transparent image registration system which generates a subtraction image from a first X-ray transparent image captured by the X-ray tube and the X-ray detector and a digitally reconstructed radiograph generated from a treatment plan CT image, corrects the first X-ray transparent image by use of a correction image generated by processing the subtraction image in a previously specified direction, and compares the first X-ray transparent image after the correction and the digitally reconstructed radiograph to determine a movement amount of the bed.