An input to set a correction contour line inside one region from an operator is received, and a contour line of the one region is corrected so that the correction contour line becomes a part of a contour line after correction. In this case, when a start point of the correction contour line is located on a unique contour line of each region and an end point thereof is located on a contour line shared by the two regions, the contour line of the other region is maintained, and when the start point is located on the shared contour line and the end point is located on the unique contour line, the contour line of the other region is also corrected so that the correction contour line becomes the shared contour line in the contour lines of the two regions after the correction.