An UGV 1 acquires a moving image of an captured imaging area, detects a receiver of an article on the basis of frame images forming the moving image, sets, as the monitoring area, an area including at least a part of the receiver and the area at a point in time when the receiver appears, detects a outsider entering the monitoring area or a outsider heading for the monitoring area on the basis of a frame image that is temporally later than a first frame image at the point in time when the receiver appears, and executes processing for notifying the receiver of outsider detection information.