A detector includes a plurality of sensors and is configured to detect a user in preliminarily set detection areas. The sensors are arrayed horizontally at a rear portion of a vehicle such that the detection areas of the adjacent sensors overlap with each other. If any one of the sensors detects the user and outputs a detection signal (first detection signal), a threshold for a detection signal of one of the sensors adjacent to the sensor (second detection signal) is raised and the first and second detection signals are converted to first and second binary signals, respectively. A rear door is opened if the second binary signal has an output period that is contained in an output period of the first binary signal.