1. A method of visualizing loitering objects, the method comprising: detecting at least one object determined to have been in a selected area of an input image for a preset time period; obtaining representative still images of each of the detected at least one object in respective time periods during the preset time period; and performing at least one of displaying the representative still images in a time order and generating a video summary in which images of each of the detected at least one object, respectively included in the representative still images, are displayed together on a single image.