A calculation unit generates a depth image corresponding to a first three-dimensional image with respect to a region of interest and sets a first image region based on a depth gradient of the depth image. The calculation unit moves the origin of a projection line to the near side or far side in the projection line direction by an amount of movement B for a pixel in the first image region, constructs a depth image based on the moved origin of the projection line in the first image region, sets a second image region based on the depth gradient of the depth image based on the moved origin of the projection line in the first image region, sets the second image region as the first image region, and repeats these processes. Once end conditions are satisfied, the calculation unit constructs a second three-dimensional image based on the moved origin of the projection line.