1. A method for measuring a bone mineral density, by use of an x-ray image, in a bone mineral density measuring system, comprising the steps of: (a) obtaining an X-ray image of bone; (b) setting a region of interest on the obtained X-ray image of bone; (c) calculating a background trend due to soft tissues, at a bone portion within the set region of interest; and (d) calculating an index of the bone mineral density by removing the background trend due to the soft tissues, at the bone portion within the set region of interest.