Image processing method and image input device, control device, image output device and image processing system employing same

To reliably and simply prevent the output of specified image data, a unit to detect a pattern is installed in one of the devices in a system consisting of an image scanner (1), control device (2) (a personal computer) and printer (3) which are connected consecutively. This pattern detection unit contains a unit which detects a specified pattern from the mass of image data representing a document. Once this pattern has been found, either the subsequent image data are altered before being output or output of the image data is interrupted. When the system starts up, the control unit for each device checks whether that device, or one of the other devices in the system, has a pattern detection unit as described above. If it determines that no device has such a unit, it halts operation.