An external storage device is connected to an image pickup apparatus provided with a first image storage medium capable of storing plural taken images and image display means, and adapted for backup of image information stored in the first image storage medium. The device includes a second image storage medium, a connection detecting unit, a reception unit, a transfer means, a conversion unit, and a second transfer unit. The transfer unit writes predetermined taken image information read from the first image storage medium into the second image storage medium and erases the predetermined taken image information from the first image storage medium according to a detection output of the connection detecting unit and an output of the reception unit. The conversion unit converts the predetermined taken image information into an image matching a display ability of the image display means. The second transfer unit writes the converted image information in the first image storage medium.