A base-plane-height determining unit determines whether a base plane is formed with planes of different heights. A base-plane determining unit determines, when it is determined that the base plane is formed with the plural planes of different heights, whether to take the plural planes or a single plane as a basis. A height-limited-area-information creating unit creates, when it is determined to take the plural planes as the basis, height-limited-area information by taking different planes for different locations as the basis, otherwise, the height-limited-area information by taking a single plane from among the plural planes as the basis.
