Provided are an image capturing device for capturing an image in a target area, transmission parts through which an outside can be visually observed from inside of the target area and polarizing filters on an light path from the transmission parts to the image capturing device. The front- and depth-side polarizing filters relative to the image capturing device are arranged so as to have polarizing directions crossing each other. A foreign matter in the target area is detected on the basis of the image acquired by the image capturing device.