Abstract: The object of the invention is to provide a break-in detection system having excellent reliability and maintenance-free characteristics. The detection sensor 1 of FBG type detects an intruder trying to climb over the fence around premises. The detection sensor 2 of OTDR type detects an intruder trying to demolish the fence. A fiber optic cable used as a detection sensor has a sensor core wire and a LAN transmission core wire arranged in parallel to each other to form a LAN transmission channel. Said LAN transmission core wire is used to transmit videotaped image signals from ITV cameras 8 and image control signals two ways between the monitoring room 3 and the ITV control device 6. Said LAN transmission channel is available for connection by IP telephone sets 11, a LAN terminal 12, a wireless LAN terminal 14 and IP-BOX 15 for internet communication.