Acquiring radiation image data representing a radiation image of a subject photographed using a radiation exposure field aperture, acquiring information of a radiation exposure field area determined by the exposure field aperture and relating it to the radiation image data based on the information, sequentially detecting pixel data of pixels from adjacent to the boundary line of the exposure field area toward the inner side thereof in the radiation image data, sequentially determining if the values of the detected pixel data are not greater than a predetermined threshold value, terminating the detection at a point where the value of the detected pixel data is greater than the predetermined threshold value, and performing a blackening process on pixel data of a pixel outside of the exposure field area and pixel data of a pixel whose value is determined to be not greater than the predetermined threshold value.
