A first point cloud acquisition unit controls a first fixed-point three-dimensional LiDAR scanner in such a way that the first fixed-point three-dimensional LiDAR scanner performs scanning in a low-density scanning range including a monitoring target in a scanning range and generates a low-density point cloud. An alignment unit aligns the low-density point cloud and known three-dimensional shape data about the monitoring target with each other. A scanning range determination unit determines a high-density scanning range including the monitoring target in a scanning range and also being narrower than the low-density scanning range, based on a result of the alignment by the alignment unit. A second point cloud acquisition unit controls the first fixed-point three-dimensional LiDAR scanner in such a way that the first fixed-point three-dimensional LiDAR scanner performs scanning in the high-density scanning range and generates a high-density point cloud.