A right infrared camera produces a right gray-scale image by imaging aiming targets disposed in a right position, and a left infrared camera produces a left gray-scale image by imaging aiming targets disposed in a left position. An average value of coordinates of the aiming targets in a preset clipping area is determined, and an average value of coordinates of the aiming targets in a preset clipping area is determined. A difference between the average value and the average value is determined, and the clipping area is moved depending on the difference.