An approaching moving object detecting unit that detects an approaching moving object existing within a predetermined range on a rear side of an own vehicle based on outputs of a periphery monitoring camera that acquires an image of the rear side of the own vehicle and a sensor that detects an approaching moving object, a relative speed detecting unit that detects relative speeds of an approaching moving object and the own vehicle, and a warning output determiner that, when an approaching moving object is detected and the own vehicle is attempting to change lane, causes a warning of a warning output unit to be generated when a relative speed detected by the relative speed detecting unit exceeds a reference relative speed with respect to which a lane change can be carried out, are provided with an object of restricting an excessive notification when notifying of the existence of an approaching moving object.