A roadside detection system installed in a vehicle to detect a roadside of a road on which the vehicle travels includes detection results acquiring means, first edge line detecting means and second edge line detecting means. The detection results acquiring means emits light waves or electromagnetic waves to a target detection region in which an object to be measured is detected and acquires an objective distance and a reflection intensity, for each of separate regions obtained by separating the target detection region into a plurality of divisions. The first edge line detecting means detects a first edge line that is a candidate of a roadside based on each objective distance. The second edge line detecting means detects a second edge line that is a candidate of a roadside based on each reflection intensity. A driver assistance system and a roadside detecting method are also provided.