Obtaining hypothetical image plot point trajectories in original image data corresponding to predetermined hypothetical image plotting trajectories on a substrate; obtaining and storing in advance hypothetical image plotting data corresponding to the hypothetical image plot point data trajectories from the original image data; selecting hypothetical image plot point data trajectories corresponding to the image plotting trajectory on the substrate when an image is plotted, and obtaining information indicating the area corresponding to the image plot point data trajectory in each hypothetical image plot point data trajectory indicated by the selected hypothetical image plot point data trajectories; identifying hypothetical image plotting data corresponding to the hypothetical image plotting data trajectories; obtaining partial hypothetical image plotting data based on the information indicating the area corresponding to the image plot point data trajectory; and obtaining image plotting data based on each partial hypothetical image plotting data.