To actualize a standby function which restores memory contents in a short time from the ON operation of a power source at the time of resume operation and to improve the operability by continuously maintaining power supply to a memory even when a power switch is turned OFF and putting the system back to its operation state according to the contents of the memory when the power switch is turned ON.

With the power SW ON, a BIOS 4 performs a suspending process with SIM input from a power controller 8. The power controller 8 stops the power supply by a battery 10 once the memory contents are all saved in response to an indication from the BIOS 4. Then when the user turns on the power SW, the BIOS 3 performs what is called the resume process by the SIM input by the power controller 8. Consequently, the system is restored to the operation state right before the power source is turned OFF to make a restart.