A detecting switch which is turned on/off in accordance with whether or not primary batteries have been attached to a primary battery holder is provided. In a normal state where no primary battery is attached, terminals (a, c) of a charge change-over switch are connected by a detection signal of the detecting switch, so that a voltage supplied from an external charger is inputted to a DC/DC converter. A secondary battery is charged by a predetermined value output of the DC/DC converter. When the primary batteries are attached, the detecting switch is turned on and terminals (b, c) of the charge change-over switch are connected, so that an output voltage of the primary batteries is inputted to the DC/DC converter. The secondary battery is charged by the predetermined value output of the DC/DC converter. Even when there is no charger, the secondary battery can be charged by the primary batteries and charging power sources are automatically switched.