PURPOSE: To reduce power consumption and power source capacity by transferring the storage contents of an auxiliary storage device to another internal storage device prior to data processing, and then turning off the power souce of the auxiliary storage device.

CONSTITUTION: A portable data processor consists of CPU1, main and auxiliary storage devices 4 and 5, etc., and the writing and reading of data to and from the auxiliary storage device 5 are controlled by an auxiliary storage controller 11. In this case, a power-on and off switch 9, FF10 for controlling it, buffer memory 13, and its control part 12 are provided. A bus multiplexer 14 switches data buses. When the power source is turned on, the FF10 is set and the auxiliary storage device is powered on by a battery 8 to transfer data read out of its to the buffere memory 13. Then, the switch 9 is released through the FF10 under a stop command to stop electrical supply to the auxiliary storage device 11.