A watermark generating unit divides an image to be processed into a plurality of regions smaller than the image. The watermark generating unit generates an inner-region image by performing first image processing on an inner image in each region of the image. The watermark generating unit performs mask processing on an outer-region pattern, and generates an outer-region image by performing second image processing on an outer image in the region. Based on the inner-region image and the outer-region image, the watermark generating unit generates a watermark image.