1. An image display method comprising the steps of: extracting a luminal or line-like three-dimensional structure from a three-dimensional image of a target object; 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; and displaying the generated projection image, wherein the step of generating the projection image is a step of generating a projection image by projecting only a part of the entirety of the portion of the three-dimensional structure located within the range of the predetermined thickness, the part continuing from a section of the three-dimensional structure intersecting the cross-sectional image within the range.