To switch projection images at high speed corresponding to the operation of rotating or and moving images and to smoothly display them.

When a projection direction or a projection position for performing projection is changed by an external operation, the resolution of the projection image within an area D under consideration on a projection plane P for performing the projection is set higher than the resolution of the projection image on the projection plane P excluding the area D under consideration, the projection plane P is divided into the small areas of different sizes inside and outside the area D under consideration according to the set resolution, the pixel values of the respective divided small areas are calculated from the pixel value of three-dimensional volume data 100 projected to the respective small areas, and the projection image is generated. After calculation of all the pixel values of the small areas for which the projection plane P is divided is ended, at least the resolution of the projection image on the projection plane other than the area D under consideration is reset gradually high and the projection image is generated.