1. An image processing apparatus for processing a radiation image, comprising: a CPU; and at least one memory operatively coupled to said CPU, said CPU and said at least one memory cooperating to function as: 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 least 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 1) a value indicating a magnitude of local dispersion of pixel values of the live image and 2) noise statistics of 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 on the subtraction image based on the emphasis degree determined by said determination unit.