PURPOSE: To provide an image forming device capable of surely preventing the print out of a data such as a paper money for preventing forgery.

CONSTITUTION: At two points of time when an input RGB signal is converted into a YUV signal by a conversion circuit 201, and when the YUV signal is processed y a DCT circuit 203 and quantized by a quantizer 204, whether or not the converted data and the quantized data are data such as a paper money stored in advance in a specific data memory 208 and for preventing forgery are discriminated through the comparison at a comparator 207. When the input signal is the forged data, the data are replaced with invalid data stored in an invalid data memory 206. Thus, the paper money the like is effectively prevented from being forged.