1. A medical image diagnosis device, comprising: an imaging unit that includes an X-ray generation unit and an X-ray detection unit; a mask image generation unit that generates a plurality of mask images that are different in terms of time by processing an image taken by the imaging unit; a first image generation unit that calculates differences between a plurality of the mask images, and extracts an image of a preceding device that is placed and kept in a blood vessel; a second image generation unit that generates a real-time fluoroscopic image depicting a situation where an insertion device is being inserted into a blood vessel, calculates a difference between the real-time fluoroscopic image and the mask image, and generates an image of the insertion device; an image synthesis unit that synthesizes an image generated by the first image generation unit and an image generated by the second image generation unit; and a display unit that displays an image generated by the image synthesis unit.