A first area and a second area are set for a first two-dimensional tomographic image where the first area and the second area overlap each other in a partial area in a horizontal direction of the first two-dimensional tomographic image. A positional deviation amount between the first two-dimensional tomographic image and a second two-dimensional tomographic image is obtained in each of the first area and the second area. A moving amount of the second two-dimensional tomographic image is determined with respect to the first two-dimensional tomographic image in an area where the first area and the second area overlap based on the positional deviation amount in the first area and the positional deviation amount the second area. Alignments are performed on the first two-dimensional tomographic image and the second two-dimensional tomographic image in an area where the first area and the second area overlap based on the moving amount.