A driving assistance device includes a camera, a moving object detector, a candidate reflection region detector, a predetermined position brightness detector and a cause determiner. The moving object detector and the candidate reflection region detector sets a detection region for detecting, from image data captured by the camera, existence of an adjacent vehicle in an adjacent lane. The predetermined position brightness detector detects a candidate light projecting object which projects light with brightness equal to or higher than a predetermined threshold value. The cause determiner determines whether or not there exists a candidate light projecting object which indicates existence of the adjacent vehicle in the detection region.