An image diagnostic apparatus includes a radiography unit which generates a mask image and contrast images before and after the injection of a contrast medium, an image memory which stores the mask image and the contrast images, an ROI identifying unit which sets a region of interest from the mask image and the contrast images, a pixel shift amount detecting unit which detects a pixel shift amount between the mask and the contrast image upon localization to the region of interest, and a processing unit which shifts at least the mask image or the contrast image in accordance with the detected pixel shift amount and performs subtraction between the images.