To evade the destruction of BIOS data on a personal computer or a device based upon a personal computer by instantaneously backing up AC input power supply generated during the erasure or writing of data to a flash ROM stored with BIOS.

If input power supply is momentary broken, a battery 6 backs up the source voltage in the personal computer or device based upon the personal computer. A battery control part 5 controls the power supply of the battery 6. A battery power supply control part 7 controls a battery control part 5 during the erasure or writing of data to the flash ROM 2 stored with the BIOS to back up the source voltage in the device. A battery power supply control signal to the battery power supply control part 7 is controlled by software. Consequently, data in the flash ROM 2 can be prevented from being destructed.