1. An image processing device, comprising: a scanner to scan an image formed in a scannable area which is allocated on a sheet; an image obtainer to obtain a processible image formed in the scannable area and scanned by the scanner; a judging unit to judge as to whether the processible image obtained by the image obtainer includes an identifying image, which qualifies a predetermined authentication criterion; and a function controller to activate a predetermined function of the image processing device, which processes the processible image obtained by the image obtainer, when the judging unit judges that the obtained processible image includes the identifying image therein.