1. An image processing apparatus comprising: a first generation unit configured to generate a first histogram of a DSA image, wherein the DSA image is a difference image between an X-ray image of a radiopaque dye injection target portion imaged after a radiopaque dye is injected and a mask image which is generated based on a pre-injection X-ray image of the radiopaque dye injection target portion imaged before the radiopaque dye is injected; a second generation unit configured to generate a second histogram of a difference image between a first X-ray image of the radiopaque dye injection target portion which is imaged as a live image before the radiopaque dye is injected and a second image which is different from the first X-ray image and is generated based on the pre-injection X-ray image; a determination unit configured to determine a pixel value, based on a difference between the first and second histograms; and a conversion unit configured to convert pixel values in the DSA image based on the value determined by the determination unit.