1. A lane departure warning system comprising: an image photographing unit attached to a front of a vehicle to photograph an object in a forward direction of the vehicle; a driving unit receiving image data from the image photographing unit, filtering the image data to search for a lane pair, converting an image coordinate of the image data into a real coordinate to calculate a lateral distance between the lane pair and the vehicle and to calculate a lateral speed of the vehicle, and generating a warning generating signal as a time of lane change elapses by obtaining the time of lane change; and a warning unit receiving the warning generating signal to generate a lane departure warning signal.