To provide a system capable of measuring a distance to an intruder, without using an exclusive device such as a distance-measuring sensor.

A monitoring system 3 includes a monitoring device 1 and a surveillance camera 2. From an image photographed by the surveillance camera 2, the monitoring device 1 creates a distance image. When the monitoring device becomes in a monitoring state, the monitoring device 1 detects an intruder from the image photographed by the surveillance camera 2. When the monitoring device detects the intruder, the monitoring device 1 subsequently identifies a grounding position on the image of the intruder. When the monitoring device 1 identifies the grounding position, the distance to the grounding position is identified by lapping the grounding position on the distance image created previously.