To evaluate an exact bone mineral density, whena reference bar region in an X-ray picture of a mandible and a reference bar disposed beside the mandible is selected, the mean value and deviation of the brightness of this region are displayed by means of letter groups. When an evaluation object region is selected, the brightness of this region is corrected according to the mean value and deviation of the brightness of the reference bar region and standard mean value and deviation, or, in other words, according to a fixed reference. The corrected brightness is displayed in the form of histogram. Furthermore, the mean value and deviation of the corrected brightness are determined, and they are displayed by means of letter groups. The bone mineral density is quantitatively evaluated from the histogram and the mean value and deviation of the corrected brightness.