There is provided an image forming device that does not require an original image scanning operation upon copying of an original for the second time or later in a device for preventing illegal copying with the use of paper fingerprint information. Upon registration of the paper fingerprint information, original image data is scanned from the original. Also, the paper fingerprint information is scanned. The image forming device associates the original image data and the paper fingerprint information with each other to register them in an information processing device. Upon copying, paper fingerprint information scanned from an original is transmitted to the information processing device to request the device to determine whether or not the paper fingerprint information has been registered. If the paper fingerprint information has been registered is received, the original image data registered in association with the paper fingerprint information is retrieved from the information processing device.