When generating projection images from three-dimensional image data, obtained by reconstructing multiple two-dimensional images, by changing the viewpoint and observing the generated projection images, facilitating the understanding of three-dimensional positional relationship. When an area of attention is specified in a first projection image and a second projection image is generated from a second viewpoint position and/or a second projection direction different from the viewpoint position and/or the projection direction of the first projection image, an area of a second projection image obtained by projecting a voxel in the three-dimensional data corresponding to the specified area of attention from the second viewpoint position and/or in the second projection direction is displayed in the second projection image.
