An image processing system for determining a workpiece pickup order in which a plurality of overlapping workpieces being transported on a conveyor are picked up by a robot, the system comprising: an imaging unit arranged to have an imaging field of view containing the conveyor, and configured to image a subject included in the imaging field of view, the imaging unit being configured to generate a luminance image representing the subject and a range image (50, 80) including height information indicating a height of at least one point on the subject from the conveyor by imaging the subject; a measurement unit configured to measure a position of each of the plurality of workpieces in the luminance image; an obtaining unit configured to define a corresponding area in the range image (50, 80) that corresponds to a workpiece area (41, 51, 72) in the luminance image for each of the plurality of workpieces in the luminance image, to generate a differential image from the range image (50, 80), and to extract stable edge points (E1-E4, E11-E14) from the differential image and configured to obtain an overlap order in which the plurality of workpieces overlap one another using the sum of the height information included in the corresponding areas in the range image (50, 80) obtained between each of the edge points (E1-E4, E11-E14) and the center (83A, 83B) of the workpiece; and a determination unit configured to determine the workpiece pickup order for the plurality of workpieces to allow a workpiece having a highest place in the overlap order to be picked up with a higher priority than other workpieces, wherein a workpiece having a higher sum has a higher place in the overlap order. an imaging unit arranged to have an imaging field of view containing the conveyor, and configured to image a subject included in the imaging field of view, the imaging unit being configured to generate a luminance image representing the subject and a range image (50, 80) including height information indicating a height of at least one point on the subject from the conveyor by imaging the subject; a measurement unit configured to measure a position of each of the plurality of workpieces in the luminance image; an obtaining unit configured to define a corresponding area in the range image (50, 80) that corresponds to a workpiece area (41, 51, 72) in the luminance image for each of the plurality of workpieces in the luminance image, to generate a differential image from the range image (50, 80), and to extract stable edge points (E1-E4, E11-E14) from the differential image and configured to obtain an overlap order in which the plurality of workpieces overlap one another using the sum of the height information included in the corresponding areas in the range image (50, 80) obtained between each of the edge points (E1-E4, E11-E14) and the center (83A, 83B) of the workpiece; and a determination unit configured to determine the workpiece pickup order for the plurality of workpieces to allow a workpiece having a highest place in the overlap order to be picked up with a higher priority than other workpieces, wherein a workpiece having a higher sum has a higher place in the overlap order.