PURPOSE: To obtain a subtraction image with high image quality by reducing the effect of beam hardening sufficiently even when a degree of the beam hardening differs from each part of an object and reducing the production of an artifact even in the edge where density changes steeply.

CONSTITUTION: Two image signals Dorg(1), Dorg(2) with a different energy state are read out of an internal memory, an unsharpened mask signal calculation means 31 calculates a mean image signal Dmean between the two image signals Dorg(1), Dorg(2), and an unsharpened mask signal Lu is calculated on an image signal by using an unsharpened mask of a rectangle longitudinally long and having a short side in a direction crossing the edge of the image. Since the mask signal Lu is based on the unsharpened mask having a short side in a direction crossing the edge, the device easily follows a steep density change in the edge and the production of an artifact in the edge is suppressed by setting a subtraction parameter on the mask signal Lu.