PURPOSE: To obtain images having excellent diagnosability to a region not irradiated with radiations by discriminating a subject part formed by transmission of the radiations through a subject, a through part formed without being shielded by the subject and a shielding part not irradiated with the radiations, extracting the subject image data and subjecting the data to image processing.

CONSTITUTION: The frequencies of pixel values are successively compared from the through part side of the large image values for the previously read images by the histogram of the digital image data on the breast obtd. by previous reading. Consequently, there are the through part of a steep one peak and the pixel value region of broad width. These parts are the skin. There are then the lungfield, the cardiodiaphragm and the mediastinal septum in order of the larger pixel values. The final peak is the part of a radiation protector. The pixel value regions to be subjected to image processing, etc., including the through part and the subject transmission part are selected by detecting these peaks. The regulation of the tube voltage of a photoelectric converter prior to normal reading and the gain adjustment of an amplifier before input to an A/D converter are executed.