An imaging unit has its imaging field of view containing the conveyor, and images a subject in the imaging field. The imaging unit generates a luminance image representing the subject and a range image including height information indicating a height of at least one point on the subject from the conveyor by imaging the subject. A measurement unit measures a position of each workpiece in the luminance image. An obtaining unit obtains an overlap order of the workpieces based on the height information in the range image corresponding to the position of each workpiece in the luminance image. A determination unit determines the workpiece pickup order to allow a workpiece having a highest place in the overlap order to be picked up with a higher priority than other workpieces.