A display method and apparatus of an X-ray projection image for medical use wherein a projection interested area to construct the X-ray projection image of an object is set on an image layer with an extending predetermined thickness perpendicular to an X-ray radiation plane defined by rotational radiation of an X-ray beam, the projection interested area is comprised of the three dimensional X-ray absorption coefficient data, and the X-ray projection image is produced by projecting the three dimensional X-ray absorption coefficient data existing in the projection interested area with respect to a projection plane intersecting the X-ray radiation plane.