GRADATION CONVERSION PROCESSING

In gradation conversion of a radiographic image, a pixel value range in an object area as a pixel value width, a representative pixel value of the object, and a curved line of the gradation conversion such that the pixel value width is adjusted to a predetermined width and the representative pixel value is adjusted to a predetermined pixel value, are extracted, and a gradation converting step is performed, of performing conversion on the image, based on the curved line of the gradation conversion.