1. An image surveillance system comprising: an image acquiring unit which receives a current input image; and an image detecting unit which receives and processes the current input image, and comprises: a foreground detecting unit which detects a foreground region based on a difference between pixel information about a background image and pixel information about the current input image; a still region detecting unit which detects a candidate still region by clustering foreground pixels of the foreground region and determines whether the candidate still region is a falsely detected still region or a true still region by using information about the candidate still region; and an object detecting unit which determines whether an object is left behind or taken away, based on edge information about the true still region, wherein the object detecting unit determines that the object is left behind if an edge intensity of the true still region in the current input image increases compared to an edge intensity of the background image in a previous input image, and wherein a determining unit determines that the object is taken away if the edge intensity of the true still region in the current input image decreases compared to the edge intensity of the background image in the previous input image.