To monitor and control a battery even if a battery control circuit in the battery pack stops owing to the overdischarging of the battery.

The battery pack 5 is provided with the battery control circuit 7 which operates with the discharging output of the battery, a charger output to control the inside of the battery, an overdischarging preventing circuit 8 which stops the discharging output of the battery so as to prevent the battery from being overdischarged, the battery 6, a charger 2 which charges the battery pack 5, and a host control circuit 3 which controls the inside of a PC main body 1. The host control circuit 3 and battery control circuit 7 are connected by a bus 9 in a communicable state and the battery 6 can be monitored and controlled by the host control circuit 3. When the discharging output of the battery 6 stops, the subordinate power supply circuit 12 supplies the electric power to the battery control circuit 7.