1. An optical monitoring system, comprising: a moving body detection sub-system that images a monitored region onto an image plane and detects a moving body from changes over time in the monitored region; a speed detection sub-system that detects a speed of the moving body in the image plane; a scale detection sub-system that detects a size for the moving body in the image plane; and a moving body estimation sub-system that decides whether the moving body is a predetermined monitored subject based on the speed detected by the speed detection sub-system and the size detected by the scale detection sub-system.