1. An X-ray image diagnosis apparatus comprising: an imaging unit, implemented by processing circuitry, 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 configured to store the X-ray image of the given region captured by the imaging unit; an image processing unit, implemented by the processing circuitry, 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; a display configured to display the synthesized image created by the image processing unit; an X-ray diaphragm configured to focus X rays; and a control unit, implemented by the processing circuitry, configured to control the X-ray diaphragm to change the region of interest captured by the imaging unit from a first region of interest to a second region of interest, wherein when the region of interest is changed from the first region of interest to the second region of interest, the image processing unit creates the synthesized image by using an X-ray image of the second region of interest, an X-ray image of the first region of interest excluding an intersection between the second region of interest and the first region of interest, and an X-ray image of a peripheral region excluding a union between the first region of interest and the second region of interest.