To measure the position of an intruder with a simple constitution in an intrusion detection system for detecting a human action using spatial feature amount.

In an intrusion detection system 201, distance measuring equipment 101 includes; a transmitter 11 for measurement to transmit acoustic waves; a receiver 12 for measurement which is arranged in the vicinity of the transmitter 11 or is integrated with the transmitter 11 to receive the acoustic waves transmitted from the transmitter 11; and a control section 13 for acquiring timing information showing arrival timing of a plurality of reception waves to be acoustic waves arriving in the receiver 12 directly or due to reflection from the transmitter 11 and amplitude information showing amplitude of each reception wave before the human action is detected by an intrusion detection device 151 and after the detection and calculating a distance between the distance measuring equipment 101 and a human being in a prescribed area based on each of the acquired timing information and each of the acquired amplitude information.