A metal housing (1) has an aperture (1a). A first conductor (3a) is disposed in a portion except for the aperture (1a) of the metal housing (1), and is capacitively coupled to the metal housing (1). A second conductor (3b) is a conductor transparent to visible light, and is disposed in the aperture (1a) of the metal housing (1). An AC voltage is applied to the first conductor (3a) and the second conductor (3b).