An added information encoding part repeatedly arranges two kinds of copy inhibition code arrays of the unit two-dimensional arrays to which copy inhibition codes are assigned, and condition code arrays of the unit two-dimensional arrays to which codes for removing the copy inhibition are assigned, into a two-dimensional array according to given rules. A background image formation part generates a background image in which copy inhibition pattern image areas and condition pattern image areas are repeatedly and two-dimensionally arranged according to given rules, while referring to this array, and the pattern images corresponding to respective codes stored in the background image formation part, and stores the generated background image into a background image buffer. An image composing part reads out document images from a document image buffer, and the background image from the background image buffer, and superimposes them into a composite image.