To provide a supervisory system provided with a configuration capable of surely storing many required video images as files.

The supervisory system 1 is provided with: a sensor 2 for sensing a mobile body intruded into a prescribed range; a camera mechanism section 3 for photographing a video image at least within a sensing range of the sensor 2; a body unit 4 for processing a sensing signal of the sensor 2 and a photographed video signal; and a semiconductor memory medium 5 for recording the photographed video image on the basis of the sensing signal of the sensor 2. The semiconductor memory medium 5 removable from the body unit 4 records recording address information of a file to be recorded next and erasure address information of a file to be deleted next.