To prevent a locking device from forgetting about locking of a double sliding door.

In the case a hook main body 41B of a locking device is interlocked with a holding seat 42 to become a state of locking, a pair of electric contacts 51 and 52 provided to the holding seat 42 are connected with the hook main body 41B to combine a switch device 5 turned ON with the locking device provided to a double sliding door. The electric contacts 51 and 52 are connected to a sensor loop or an invader monitoring panel, and locking and unlocking of the locking device are discriminated by the invader monitoring panel.