An intrusion detection system includes an intrusion detection device, and a pair of leaky transmission paths including a leaky transmission path of a sending side which is connected to the intrusion detection device and outputs a leaky electric wave on the basis of an output from the intrusion detection device and a leaky transmission path of a receiving side which receives the leaky electric wave output from the leaky transmission path of the sending side, and the intrusion detection device detects that the leaky electric wave received in the leaky transmission path of the receiving side varies to detect the presence or absence of intrusion and an intrusion point.