To attain efficient power saving by variably (for several-hours) setting up a power supply control part to a power saving state in accordance with the residual of a battery when a system part is in a power saving state (normal suspending state) so that a current amount uselessly consumed for periodical (a several-second unit) restoration to a normal state to detect a low battery state in a conventional method can be saved.

When the system receives remaining operation time in a 'normal suspending' state and starts the 'normal suspending' state, time obtained by adding the remaining operation time to current time is set up in a real time clock(RTC) 9 and a power supply controller 8 starts a power saving state. When the controller 8 is restored to a normal state by an interruption from the RTC 9, whether a battery is a low battery state or not is discriminated, and at the time of detecting the low battery state, the system is restored to the normal state and executes disk storing suspension.