An X-ray measuring instrument in which an object under examination is irradiated with X-rays, measurement data on the object is detected, a filter for adjusting the amount of transmitted X-rays is disposed between an X-ray source and the object, the relative position of the X-ray source to the object is varied, and the acquired measurement data is computed. The measurement data is subjected to logarithm transform to acquire projection data, and the amount of absorbed X-rays of the filter corresponding to the acquired projection data is determined. The thickness of the filter is computed by using a predetermined transform formula for the acquired amount of absorbed X-rays. A correction coefficient corresponding to the projection data acquired from the computed thickness of the filter is determined, and the projection data is multiplied by the determined correction coefficient. The projection data multiplied by the correction coefficient is restructure-computed to obtain a three-dimensional image.