To avoid leveling down of a system and a loss of data during work, by supervising a residual amount of a plurality of batteries, in the case of the battery residual amount decreasing less than a reference when a battery is removed, locking this battery.

When power supply is performed from a battery 2A, a battery information control device 2C acquires a residual amount of batteries 2A, 2B periodically from a battery information transmitting device 2D. Here, the battery 2A is removed, even only the rest of the battery 2B is decided for whether it leads or not to a fixed reference value. When the battery is less than the reference value, a lock signal is fed relating to a battery lock mechanism 2Ga. When the battery is more than the reference value, a lock release signal is fed. Also relating to the battery 2B, similar operation is performed.