1. An X-ray measuring instrument comprising: an X-ray source that generates X-rays to be irradiated to an object; an X-ray detector that is opposed to the X-ray source with the subject between them and detects X-rays transmitted by the object as measurement data; a filter that is interposed between the X-ray source and the object and regulates an exposure of X-rays to be transmitted; a holding unit that holds the X-ray source and the X-ray detector; a rotating unit that rotates the X-ray source and the X-ray detector about the object; and a control processing unit that computes measurement data items which are detected by the X-ray detector at a plurality of angles with respect to the object rotated by the rotating unit; wherein, the control processing unit logarithmically converts the measurement data so as to produce projection data, obtains an X-ray absorption coefficient relevant to the filter using the produced projection data, calculates the thickness of the filter by applying a predetermined conversion expression to the obtained X-ray absorption coefficient, obtains a correction coefficient for the produced projection data according to the calculated thickness of the filter, multiplies the projection data by the obtained correction coefficient, computes the projection data, which is multiplied by the correction coefficient, for reconstruction, and thus produces a three-dimensional image.