To calculate ground levels at positions other than prescribed ground level survey points, based on the ground levels previously surveyed in first survey at ground level survey points.

When plane surveying in conducted to acquire current geological formation of ground in a site where an underground installation is buried, ground levels surveyed at limited ground level survey point among all the survey points are stored in a memory 15. A control part 5 finds ground levels at prescribed positions by using the distances from the prescribed position to prescribed ground level survey points and the ground levels at the prescribed survey points, and prepares drawings of the underground installation, by using the found ground levels.