Data which indicates remaining capacity of a battery in terms of the terminal voltage of the battery is updated by a control unit when necessary, whereby available capacity of the battery is accurately calculated regardless of the extent of deterioration of the battery. In addition to the battery's calculated available capacity, a radio-wave receiving condition and a temperature detected by a temperature detection unit are also taken into consideration to accurately calculate and display an available time. Moreover, the control unit performs the entire process of backing up user data into a non-volatile internal memory (flash memory), which reduces power consumption required for the backup process, resulting in a longer available time.