A determining unit determines whether a background pixel value of image data matches a predetermined comparison value. A replacing unit replaces, when the determining unit determines that the background pixel value does not match the comparison value, a pixel value of a pixel of the image data with a replacing value that is obtained by subtracting the pixel value from a predetermined maximum pixel value. An encoding unit generates encoded data by encoding the image data in which the pixel value is replaced with the replacing value by using a predetermined encoding system.