An image processing circuit of an image processor performs predetermined image processes on input image data from an external device. A data scrambling circuit scrambles the processed image data in accordance with input scrambling information from a controller of an image forming system. A data scrambling circuit of the image forming system receives the scrambled input data from the image processing apparatus. A printer forms an image on the basis of the received image data. Accordingly, without changing ROMs of the image processor, a scrambling function is arbitrarily changed by the image forming system.