To accurately predict the remaining usable time corresponding to the use status in real-time, and to inform exact usable time information by monitoring the use status of a battery.

A CPU 101 finds a first cumulative battery consumption, based on an operating time of each operating mode by referring to a current consumption table stored in a storage part 106. A second cumulative battery consumption at a reference time point is calculated from a difference between a battery voltage at the time point and a battery voltage at the time of previous battery charging stop by referring to a battery remaining quantity table. When the difference between the first cumulative battery consumption and the second cumulative battery consumption is within a predetermined range, an average consumption current is calculated based on the first cumulative battery consumption and the operating time. The remaining usable time is calculated and displayed on a display unit 105, based on the battery remaining quantity at the time point and the average consumption current obtained by referring to the battery remaining quantity table.

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