1. An image diagnostic apparatus comprising: a radiography unit which generates a mask image before injection of a contrast medium and a plurality of contrast images 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 plurality of contrast images; a region setting unit which sets a region of interest in a subtraction image from the mask image and the plurality of contrast images; a pixel shift amount detecting unit which detects a pixel shift amount between the mask image and the plurality of contrast images upon localization to the set region of interest, wherein the pixel shift amount detecting unit detects a pixel shift amount for each of the plurality of contrast images, or detects a pixel shift amount common to the plurality of contrast images; and a pixel shift processing unit which shifts at least one of the mask image and the plurality of contrast images in accordance with the detected pixel shift amount and performs subtraction between the images, wherein the region setting unit sets, as the region of interest, a motion artifact occurrence region remaining in a subtraction image between the mask image and the plurality of contrast images.