An image processing device stores a main program that controls basic operations that the device conducts as a copying machine, a printer, a facsimile machine, etc., as well as a sub program that controls additional operations that the device conducts as a net work scanner. When a user purchases an additional function pack and sends a production number of the image processing device and ID information of the pack to a managing device, the managing device returns a releasing key produced by encoding the production number. A control section of the device decodes the releasing key, and permits the foregoing access if the production number obtained by the decoding matches that stored in the memory section. Therefore, even after shipment, extension of functions can be smoothly implemented at lower costs without replacement of boards or memories. Thus, an image processing device realized with a digital complex machine is arranged so that extension of functions after shipment can be smoothly implemented at lower costs.