1. An image diagnostic apparatus comprising: a radiography unit which generates a mask image before injection of a contrast medium and a contrast image after injection of the contrast medium by radiographing a subject before and after injection of the contrast medium; an image storage unit which stores the mask image and the contrast image; a region setting unit which sets a region of interest from the mask image and the contrast image; a pixel shift amount detecting unit which detects a pixel shift amount between the mask image and the contrast image upon localization to the set region of interest; and a subtraction processing unit which shifts at least one of the mask image and the contrast image in accordance with the detected pixel shift amount and performs subtraction between the images.