1. A sensitivity correction coefficient calculating system for an X-ray detector having a detection surface where detection elements for detecting an X-ray intensity are aligned one-dimensionally or two-dimensionally, characterized in that an X-ray source, where fitting of the X-ray intensity distribution is possible with an approximation function for the X-rays with which said detection surface is irradiated, is used to carry out function fitting on the measured X-ray intensity detected for each detection element, and the sensitivity correction coefficient for each detection element is found from the ratio of the calculated X-ray intensity calculated from the fitted approximation function to the measured X-ray intensity.