PURPOSE: To form a radiograph extracting a bone part tissue without being affected by the thickness of an object to be photographed and with high accuracy by controlling a tube voltage so that the ratio of a density variation of the bone part tissue by beam hardening of a radiant ray of high energy and low energy is nearly equal to a ratio of a subtraction coefficient.

CONSTITUTION: A tube voltage of each radiant ray is controlled so that a ratio of a density variation (a) of a bone part tissue by beam hardening of a radiant ray of high energy and a density variation (b) of the bone part tissue by beam hardening of a radiant ray of low energy becomes roughly equal to a ratio of a subtraction coefficient, each radiant ray is allowed to irradiate two pieces of accumulative phosphor sheets A, B and a radiation image of an object 1 to be photographed is accumulated and recorded. By reading out photoelectrically this accumulated and recorded radiograph, a digital image signal is obtained, and by executing subtraction between the corresponding picture elements, a difference signal for forming an image of only the bone part tissue is obtained. As a result, the influence of beam hardening is reduced, and a bone salt can be quantitatively analyzed with high measuring accuracy.