To identify pixels constituting image elements for generating a labeled image in which pixels are labeled with identification information, a pixel block including four pixels adjacent to one another in two dimensions is inputted as a unit from data including pixels that form an image. All of the on-pixels that are subject for grouping, included in the pixel block are labeled with the same identifier. Since the on-pixels that are included in a pixel block are consecutively connected, such pixels can be labeled with the same identifier without having to calculate whether such pixels are connected.