A pattern generating section forms two patterns based on a pattern size, an embedding strength, and a pattern attenuation ratio. When corresponding pixels in two patterns are added to each other, all of elements become zero. When all of pixels in each of patterns are added to each other, the added pixels become zero. Each of patterns has two edges, which include discontinuous pixel values and pass through a center portion of each of patterns in difference directions from each other. Absolute values of pixel values of the patterns are the largest at a center thereof. The absolute values of pixel values of the patterns decrease as being away from the center thereof. A pattern selecting section selects one of the two patterns in response to additional information. A pattern superimposing section superimposes the additional information on the image data to embed it into the image data.