An image processing device, comprising: image data acquisition means for acquiring image data; histogram generation means for generating, based on pixel values of an image indicated by the image data acquired by the image data acquisition means, a histogram for each region in the image; region selection means for determining whether each region presents a gradation based on the histogram for each region and selecting from the image a region that does not present the gradation in the histogram generated by the histogram generation means; and image processing means for embedding a watermark in the region selected by the region selection means, wherein: the region selection means determines that the region presents the gradation when a number of gradation values in the histogram each having a frequency equal to or more than a threshold is less than a predetermined number, and determines that the region does not present the gradation when the number is equal to or more than the predetermined number; or the region selection means determines that the region presents the gradation when the number of the gradation values in the histogram each having a frequency equal to or more than a threshold is equal to or more than a predetermined number, and determines that the region does not present the gradation when the number is less than the predetermined number; or the region selection means determines that the region presents the gradation when a distance between the gradation values in the histogram each having a frequency equal to or more than a threshold is shorter than a predetermined distance, and determines that the region does not present the gradation when the distance is equal to or longer than the predetermined distance. image data acquisition means for acquiring image data; histogram generation means for generating, based on pixel values of an image indicated by the image data acquired by the image data acquisition means, a histogram for each region in the image; region selection means for determining whether each region presents a gradation based on the histogram for each region and selecting from the image a region that does not present the gradation in the histogram generated by the histogram generation means; and image processing means for embedding a watermark in the region selected by the region selection means, wherein: the region selection means determines that the region presents the gradation when a number of gradation values in the histogram each having a frequency equal to or more than a threshold is less than a predetermined number, and determines that the region does not present the gradation when the number is equal to or more than the predetermined number; or the region selection means determines that the region presents the gradation when the number of the gradation values in the histogram each having a frequency equal to or more than the region selection means determines that the region presents the gradation when a number of gradation values in the histogram each having a frequency equal to or more than a threshold is less than a predetermined number, and determines that the region does not present the gradation when the number is equal to or more than the predetermined number; or the region selection means determines that the region presents the gradation when the number of the gradation values in the histogram each having a frequency equal to or more than a threshold is equal to or more than a predetermined number, and determines that the region does not present the gradation when the number is less than the predetermined number; or the region selection means determines that the region presents the gradation when a distance between the gradation values in the histogram each having a frequency equal to or more than a threshold is shorter than a predetermined distance, and determines that the region does not present the gradation when the distance is equal to or longer than the predetermined distance. determines that the region does not present the gradation when the number is less than the predetermined number; or the region selection means determines that the region presents the gradation when a distance between the gradation values in the histogram each having a frequency equal to or more than a threshold is shorter than a predetermined distance, and determines that the region does not present the gradation when the distance is equal to or longer than the predetermined distance.