An image processing apparatus storing an image data to an external memory medium connected to the image processing apparatus is provided. Where there is not enough usable capacity in the external memory medium, the image processing apparatus generates medium identification information and stores the generated medium identification information to the external memory medium. After a user detaches the external memory medium from the image processing apparatus and uses his or her computer to delete or move unnecessary files in the external memory medium to increase the usable capacity therein, the user attaches the external memory medium to the image processing apparatus again. At this moment, the external memory medium is authenticated by the medium identification information stored therein. Thus, this authentication performed using the medium identification information prevents a third person from obtaining the image data of the user while the user is away from the image processing apparatus.