A monitoring device comprising: - a current data computing unit for obtaining pieces of distance data representing distances to a plurality of physical objects present in a monitoring region, from measurement results obtained by a three-dimensional laser scanner, and using the distance data as current distance data, the three-dimensional laser scanner measuring the monitoring region; - a comparison data computing unit for obtaining past distance data from the measurement results, and converting the past distance data into comparison distance data; - a changed-region extracting unit for calculating difference values between the current distance data and the comparison distance data, and extracting changed regions whose difference values are greater than or equal to a threshold; - a front image creating unit for extracting grids corresponding to the changed regions from grids in the image represented by the current distance data, and for creating a frontal viewpoint image including only the extracted grids; - a coordinate transforming unit for creating an image obtained by performing a coordinate transformation on the frontal viewpoint image into a directly-above viewpoint image; the monitoring device further comprising: - a directly-above image labeling unit for putting adjacent pixels together into one aggregate among pixels included in the changed regions shown in the image created by the coordinate transforming unit, and assigning labels on an aggregate-by-aggregate basis; and - a front image labeling unit for putting adjacent pixels together into one aggregate among the pixels included in the changed regions shown in the frontal viewpoint image, and assigning labels on an aggregate-by-aggregate basis, wherein a label dividing unit (110, 1 10a) reassigns, using the labels assigned by the directly-above image labeling unit, the labels assigned, by the front image labeling unit, to the respective pixels included in the changed regions shown in the frontal viewpoint image, such that different labels are assigned to the different physical objects, and wherein an object identifying unit identifies the plurality of physical objects present in the monitoring region, on a basis of the labels reassigned by the label dividing unit (110, 110a). - a current data computing unit for obtaining pieces of distance data representing distances to a plurality of physical objects present in a monitoring region, from measurement results obtained by a three-dimensional laser scanner, and using the distance data as current distance data, the three-dimensional laser scanner measuring the monitoring region; - a comparison data computing unit for obtaining past distance data from the measurement results, and converting the past distance data into comparison distance data; - a changed-region extracting unit for calculating difference values between the current distance data and the comparison distance data, and extracting changed regions whose difference values are greater than or equal to a threshold; - a front image creating unit for extracting grids corresponding to the changed regions from grids in the image represented by the current distance data, and for creating a frontal viewpoint image including only the extracted grids; - a coordinate transforming unit for creating an image obtained by performing a coordinate transformation on the frontal viewpoint image into a directly-above viewpoint image; the monitoring device further comprising: - a directly-above image labeling unit for putting adjacent pixels together into one aggregate among pixels included in the changed regions shown in the image created by the coordinate transforming unit, and assigning labels on an aggregate-by-aggregate basis; and - a front image labeling unit for putting adjacent pixels together into one aggregate among the pixels included in the changed regions shown in the frontal viewpoint image, and assigning labels on an aggregate-by-aggregate basis, - a directly-above image labeling unit for putting adjacent pixels together into one aggregate among pixels included in the changed regions shown in the image created by the coordinate transforming unit, and assigning labels on an aggregate-by-aggregate basis; and - a front image labeling unit for putting adjacent pixels together into one aggregate among the pixels included in the changed regions shown in the frontal viewpoint image, and assigning labels on an aggregate-by-aggregate basis, wherein a label dividing unit (110, 1 10a) reassigns, using the labels assigned by the directly-above image labeling unit, the labels assigned, by the front image labeling unit, to the respective pixels included in the changed regions shown in the frontal viewpoint image, such that different labels are assigned to the different physical objects, and wherein an object identifying unit identifies the plurality of physical objects present in the monitoring region, on a basis of the labels reassigned by the label dividing unit (110, 110a).