A diagnosis assisting apparatus, comprising: volume data storing means, for storing volume data (7a, 7b) constituted by data obtained by imaging a subject; overview image display means, for generating overview images (8, 9) of tubular hollow structures that constitute the subject based on the volume data (7a, 7b), and for displaying the overview images (8, 9) on a screen, the overview images (8, 9) including at least one of an extension expanded image and an extended sectional image, which have been generated from the volume data (7a, 7b); the extension expanded image representing the tubular hollow structure, which is linearly extended and then cut open along the core line extracted from the tubular hollow structure, and the extended sectional image linearly extending the hollow tubular structure and then cutting it into cross sections along a core line extracted from the hollow tubular structure; target point setting means, for setting, as target points (12, 13), at least one point specified within the overview images (8, 9), and at least one point within the volume data (7a, 7b) corresponding to the at least one point specified within the overview images, the at least one point within the volume data being obtained based on the relationship between the overview images (8, 9) and the volume data (7a, 7b); target volume setting means, for setting a target volume that includes the target point within the volume data (7a, 7b); line of sight setting means, for setting a line of sight vector within the volume data (7a, 7b) having the target point as an endpoint and a movable viewpoint as a starting point, and for changing the direction of the line of sight vector by moving the viewpoint; and detailed image display means, for projecting the target volume onto a projection plane perpendicular to the direction of the line of sight vector to generate at least one detailed image that represent details of the structure in the vicinity of the target point in the volume data (7a, 7b), and for displaying the at least one detailed image on the screen, characterized in that a target volume memory is provided, adapted to store a target volume having a predefined shape and size; the line of sight setting means detects, as a movement of viewpoint, a movement of an input device on the screen where the at least one detailed image is displayed, and determines a new direction of the line of sight vector based on the amount of movement of the input device, and the detailed image display means re-generates the at least one detailed image based on the new direction of the line of sight vector; and wherein the detailed image display means displays the at least one detailed image overlapped with the overview images (8, 9) at the positions of the overview images (8, 9) at which the target point is set. volume data storing means, for storing volume data (7a, 7b) constituted by data obtained by imaging a subject; overview image display means, for generating overview images (8, 9) of tubular hollow structures that constitute the subject based on the volume data (7a, 7b), and for displaying the overview images (8, 9) on a screen, the overview images (8, 9) including at least one of an extension expanded image and an extended sectional image, which have been generated from the volume data (7a, 7b); the extension expanded image representing the tubular hollow structure, which is linearly extended and then cut open along the core line extracted from the tubular hollow structure, and the extended sectional image linearly extending the hollow tubular structure and then cutting it into cross sections along a core line extracted from the hollow tubular structure; target point setting means, for setting, as target points (12, 13), at least one point specified within the overview images (8, 9), and at least one point within the volume data (7a, 7b) corresponding to the at least one point specified within the overview images, the at least one point within the volume data being obtained based on the relationship between the overview images (8, 9) and the volume data (7a, 7b); target volume setting means, for setting a target volume that includes the target point within the volume data (7a, 7b); line of sight setting means, for setting a line of sight vector within the volume data (7a, 7b) having the target point as an endpoint and a movable viewpoint as a starting point, and for changing the direction of the line of sight vector by moving the viewpoint; and detailed image display means, for projecting the target volume onto a projection plane perpendicular to the direction of the line of sight vector to generate at least one detailed image that represent details of the structure in the vicinity of the target point in the volume data (7a, 7b), and for displaying the at least one detailed image on the screen, characterized in that a target volume memory is provided, adapted to store a target volume having a predefined shape and size; the line of sight setting means detects, as a movement of viewpoint, a movement of an input device on the screen where the at least one detailed image is displayed, and determines a new direction of the line of sight vector based on the amount of movement of the input device, and the detailed image display means re-generates the at least one detailed image based on the new direction of the line of sight vector; and wherein the detailed image display means displays the at least one detailed image overlapped with the overview images (8, 9) at the positions of the overview images (8, 9) at which the target point is set.