PURPOSE: To prolong time for operations due to the battery of the lap top type computer.

CONSTITUTION: When it is detected by a key input signal 101 from a keyboard 1 that a key input and a special key input are turned off or it is detected by an operating signal 103 from a system constituting device 3 that a real processing is not executed, a control circuit 2 outputs a control signal 102 to a voltage varying circuit 4. Corresponding to the control signal 102 from the control circuit 2, the voltage varying circuit 4 supplies battery power supply 105 from a battery pack 5 to the system constituting device 3 in the state of a low voltage.