1. An X-ray medical image processing apparatus comprising: an image inverting unit that generates a first inverted image obtained by inverting a first medical image in a left-and-right direction of an examined subject and generates a second inverted image obtained by inverting a second medical image that is different from the first medical image in the left-and-right direction of the examined subject; a displacement detecting unit that detects a displacement between the first medical image and the first inverted image; a registration unit that, based on the displacement that has been detected by the displacement detecting unit, generates one of a corrected image obtained by correcting the second medical image and a corrected inverted image obtained by correcting the second inverted image; and a difference image generating unit that generates one of a difference image between the second inverted image and the corrected image and a difference image between the second medical image and the corrected inverted image.