a simple outdoor security system which uses sensors to monitor an area around a building to detect intruders while reducing the number of sensors necessary for ensuring detection accuracy , which has at least a detection means for radiating light or an electric wave to a prescribed area to detect an object , receiving a wave reflected by the object and detecting at least the speed and location of the object ; an imaging means for imaging the object ; and a means for directing said imaging means toward the object according to the object &# 39 ; s speed and location detected by said detection means .