Provided is a bed positioning system 100 for a radiation therapy system 3 capable of capturing X-ray transparent images 200 and CT images by rotating an X-ray tube 4 and an X-ray detector 5 around a subject 1 on a bed 2, comprising: a transparent image registration system 12 which generates a subtraction image 220 from a first X-ray transparent image 200 captured by the X-ray tube 4 and the X-ray detector 5 and a digitally reconstructed radiograph 210 generated from a treatment plan CT image, corrects the first X-ray transparent image 200 by use of a correction image 230 generated by processing the subtraction image 220 in a previously specified direction, and compares the first X-ray transparent image 200 after the correction and the digitally reconstructed radiograph 210 to determine a movement amount of the bed 2.
