1. A region determination device for a slice model, the region determination device dividing a region of the slice model into a printing region to be printed and a non-printing region not to be printed, the slice model being obtained as a result of slicing a three-dimensional model of a three-dimensional printing object at a predetermined interval and being defined by contours representing profiles of the three-dimensional printing object, wherein the contours include an island contour representing a shape of an outer profile of the printing region of the three-dimensional printing object and a hole contour representing a shape of a hole in the three-dimensional printing object, the region determination device comprising: a storage processor configured or programmed to store the slice model; a target contour setting processor configured or programmed to extract a predetermined contour from the contours of the slice model stored by the storage processor and set the extracted predetermined contour as a target contour; a first contour determination processor configured or programmed to determine whether the target contour set by the target contour setting processor is the island contour or not; a closest enclosing contour determination processor configured or programmed to, in a case in which the target contour is determined by the first contour determination processor to be the island contour, determine whether a closest enclosing contour enclosing the target contour at a closest position thereto has been set or not; a second contour determination processor configured or programmed to, in a case in which the closest enclosing contour is determined by the closest enclosing contour determination processor to have been set, determine whether the closest enclosing contour is the island contour or not; a deletion processor configured or programmed to, in a case in which the closest enclosing contour is determined by the second contour determination processor to be the island contour, deletes the target contour from the slice model and stores, on the storage processor, a slice model obtained as a result of deleting the target contour from the slice model; and a region determination processor configured or programmed to divide the region of the slice model stored on the storage processor into the printing region and the non-printing region.