Extracting a luminal or line-like three-dimensional structure from a three-dimensional image of a target object, then generating a projection image by projecting a portion of the entirety of the three-dimensional structure on a cross-sectional image of the target object intersecting the three-dimensional structure, the portion being a portion located within a range of a predetermined thickness from a cross-section represented by the cross-sectional image or a portion located within a range of a predetermined distance from a position where the three-dimensional structure is intersecting the cross-sectional image, and displaying the generated projection image.