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 displaying the representative still images in a time order in a screen, 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 in another screen, wherein the generating the video summary comprises displaying a moving trajectory indicating a path of the object by using the representative still images obtained in respective time periods with respect to the object, wherein the displaying the representative still images comprises displaying, in the time order, the representative still images along with time information which respectively indicates time points when the representative still images are captured by a camera, and wherein the generating the video summary comprises displaying the video summary in which the representative still images along with time information which respectively indicates time points when the representative still images are captured by a camera are displayed, the representative still images along with time information displayed in the video summary in another screen being the same as the representative still images along with time information displayed in the time order in the screen.