PURPOSE: To completely guarantee a backup operation against removal, of a battery pack at the time of operating a body by providing a battery pack mount/removal detecting pin between the body and the pack.

CONSTITUTION: When an operator intends to remove a battery pack, a battery pack mount/removal detecting pin 4 is disconnected before a positive side power source contact pin 2 and a negative side power source contact pin 3 are removed since an effective engaging length of the pin 4 is short, and a voltage drop across a resistor 5 becomes zero. An input of a buffer 6 becomes a low level, and an output of the buffer 6 becomes a low level. A backup controller 7 designates a backup operation to a circuit to be controlled according to the signal state. In this case, since the pins 2, 3 are not disconnected, a power source voltage from a battery 1 is completely supplied.