PURPOSE: To cool a face plate and prevent the quality of a picture from falling due to the clinging of dust, by filling gas lighter than air, in an enclosed space in front of theface plate.

CONSTITUTION: The temperature of a fluorescent film 6 and a face plate 2 rises due to the collision of an electron beam. Heat generated by the collision is taken away by gas filled in an enclosed space 10. The cooling effect of the gas is inversely proportional to the square root of its molecular weight. When helium is filled in the enclosed space 10, the helium transfers the heat to a cylindrical part 7 or a pane 9 at capability several times higher than air, so that the face plate 2 is enough cooled spontaneously by the atmosphere. Since the face plate 2 is out of contact with the atmosphere and the pane 9 located in contact with the atmosphere is separated from the face plate, dust does not electrostatically cling to deteriorate the quality of the picture.