1. A dynamic range compression method for a radiographic image, for obtaining a processed image signal Sproc having a dynamic range which is narrower than a dynamic range of an original image signal Sorg, the method comprising the steps of: obtaining the original image signal Sorg representing the original image based on a radiation transmitted through an object; obtaining an unsharp mask signal Sus corresponding to said original image signal Sorg; detecting an information of said object; determining a correction value f1 (Sus) which is a function that decreases as said unsharp mask signal Sus increases in accordance with said information of said object; and conducting an operation based on an equation: EQU Sproc=Sorg+f1 (Sus)