When generating a projection image which is an image formed of image information on a plurality of visual lines viewing a first structure in a three-dimensional medical image from a given viewpoint being projected on a given projection plane, setting an assumed position of a treatment tool in the three-dimensional image, detecting a surface of the first structure from the three-dimensional medical image, identifying a second structure in the three-dimensional medical image located at a position masked by the first structure, detecting an intersection point between an assumed path formed when the treatment tool is moved from the assumed position towards the second structure and the surface of the first structure, and displaying the intersection point in an identifiable manner at a position to which image information on a visual line connecting the viewpoint and the intersection point is projected.
