1. An image processing apparatus for processing a radiation image, comprising: a subtraction unit configured to generate a subtraction image by performing subtraction between a mask image serving as a radiation image obtained by capturing an object at, at least of a specific region of which does not include contrast medium, and a live image serving as a radiation image obtained by capturing the object which includes the contrast medium; a determination unit configured to determine, based on at least one of the mask image and the live image, an emphasis degree serving as a degree of emphasis processing for the subtraction image; and a processing unit configured to perform the emphasis processing for the subtraction image based on the emphasis degree.