PURPOSE: To remove influence of noise to an internal circuit by providing a switch for switching from a battery to a charging terminal or an internal circuit, and providing a switch for switching from the internal circuit to the battery or an external terminal.

CONSTITUTION: A first switch 3 for switching from a battery 5 to a charging terminal 2 or an internal circuit 6, and a second switch 4 for switching from the circuit 6 to an external power source terminal 1 and the battery 5, are provided. When the external power source is not input, the switches 3, 4 are so set as to connect the battery 5 to the circuit 6. When the external power source is input, the switch 3 is so set as to connect the terminal 2 to the battery 5 and the switch 4 is so set as to connect the terminal 1 to the circuit 6. Thus, influence of noise to the circuit 6 can be removed.