To prevent a door from being set to a shutoff state even when a battery is exhausted in an auxiliary lock body operated by the built-in battery.

The auxiliary lock body is provided with a first voltage detecting means 24 and a second voltage detecting means 25 detecting that the voltage of the battery B drops to the prescribed voltages. When the first voltage detecting means 24 detects that the voltage of the battery B drops to the prescribed voltage, it reports the case. When the second voltage detecting means 25 detects that the voltage of the battery B further drops, it operates an actuator 3 to store a bolt 1 in a lock face for unlocking before the battery B is exhausted, thereby the door is prevented from being set to the shutoff state even when the battery B is exhausted.