To provide a method and device for watching an intruder at a low cost capable of constituting a security system for watching the intruder intruding into a watching area.

A pair of illumination means 2R, 2L are disposed symmetrically on the both sides of a monitoring camera 1 centering inbetween. A parallax operation part 8 obtains illumination parallax between the 1st intruder image photographed under one side illumination means 2R, and the 2nd intruder image taken under the other illumination means 2L. A distance operation part 9 calculates the distance from the camera 1 to the intruder based on the illumination parallax.