1. An X-ray image diagnosis apparatus comprising: an imaging unit configured to capture an X-ray image of a given region of a target part of a subject in a case of obtaining a still image of the target part, and to sequentially capture an X-ray image of a region of interest inside the given region in a case of obtaining a moving image of the target part; an image storage unit configured to store the X-ray image of the given region captured by the imaging unit; an image processing unit configured to create a synthesized image by synthesizing the X-ray image of the region of interest captured by the imaging unit and an X-ray image of a peripheral region excluding the region of interest in the X-ray image of the given region stored in the image storage unit; and a display unit configured to display the synthesized image created by the image processing unit, wherein when the region of interest is changed, the image processing unit acquires an X-ray image of a changed region which is in the region of interest before the change and is changed to the peripheral region, from the X-ray image of the region of interest before the change, and then superimposes the X-ray image of the changed region on the X-ray image of the peripheral region to create the synthesized image.