An original copy sheet is pre-scanned to obtain pre-scanned image data of a bit map form with a low resolution. The pre-scanned image data is converted to JPEG thumbnail image data. An EXIF file is formed in an external media to store the JPEG thumbnail image data in the EXIF file. The original copy sheet is mainly scanned to obtain main image data of a bit map form with a high resolution. The main image data is converted to JPEG main image data. In the EXIF file in the external media, the JPEG main image data is stored.