1. An image processing apparatus comprising: a specific pattern detecting unit that detects a background pattern representing additional information from inputted original document image data; an information detecting unit that extracts the additional information represented by the background pattern which is detected by said specific pattern detecting unit; an erase processing unit that erases a prescribed region of the original document image and outputs erase-processed document image data; a re-embedding background pattern generating unit that generates a re-embedding background pattern representing the additional information extracted by said information detecting unit; a background pattern composition processing unit that embeds the re-embedding background pattern into a blank area of the erase-processed document image data and outputs re-embedded document image data; and a processor that is connected so as to transmit and receive data to/from said specific pattern detecting unit, said information detecting unit, said erase processing unit, said re-embedding background pattern generating unit, and said background pattern composition processing unit, and controls said specific pattern detecting unit, said information detecting unit, said erase processing unit, said re-embedding background pattern generating unit, and said background pattern composition processing unit.