To mix first and second digital image data at a ratio defined by pixel-unit mixing-ratio information, mixing-ratio information is embedded, in each of pixel data formed from a plurality of bits in the first digital image data, as information of more than one bit in the pixel data. The mixing-ratio information is extracted from the first digital image data. The first and second digital image data are mixed at the ratio defined by the extracted mixing-ratio information. Thus, images can be mixed together with the α blending technique without having to use any special memory such as the α memory.