display of a setting menu on the host device establishment of wireless communication between the host device and the portable external storage device and saving of setting information in the portable external storage device is the same as that shown in fig8 and a description thereof will be omitted when the user wants to know the remaining amount of battery he she requests the host device to display the remaining amount of battery by e g clicking an icon 301 upon reception of the user request the host device transmits a remaining battery amount acquisition request to the portable external storage device 302 the portable external storage device checks the remaining amount of battery and determines whether the checked remaining amount of battery is equal to or smaller than a remaining amount of battery represented by the setting information at which a warning is generated 303 if the checked remaining amount of battery is larger than the remaining amount of battery represented by the setting information at which a warning is generated i e the battery has a sufficient remaining amount the portable external storage device transmits remaining battery amount information 304 if the checked remaining amount of battery is equal to.