To monitor an auxiliary lock for sliding door glass installed at multiple positions of a building collectively at one position.

Each auxiliary lock 10 for a glass door comprises a radio transmission part 21 detecting the vibration of glass doors W11, W12 or the contact force of the glass door W12 on the inner side on a lock body 2 and transmitting alarm signals including identification information on the auxiliary lock 10 and a power supply part 27 for operating the radio transmission part 21. The alarm signals from the radio transmission part 21 of each auxiliary lock 10 are transmitted to a monitoring device 31 for monitoring the intrusion through sliding glass doors. The monitoring device 31 comprises an output circuit 36 for outputting alarm sound, alarm message or alarm light against the identification information upon receipt of the alarm signals.

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