A histogram of pixel values in a difference image between a post-injection X-ray image and composite image is generated. A histogram of pixel values in a difference image between a pre-injection X-ray image and the composite image is generated. From the difference between the histograms, information on pixel values in a region where a radiopaque dye is imaged in the difference image between the post-injection X-ray image and composite image is collected. Pixel values in the difference image between the post-injection X-ray image and composite image are converted by using the collected information, and the converted difference image is output.