PURPOSE: To absorb the expansion and contraction of a cooling medium while attempting the improvement of a heat radiating effect through the simple structure ot the tube in the title, by connecting a variable capacity vessel having its excellent heat conductivity to frame-like members which fix both of a fluorescent screen panel and a frant-face panel for being communicated with the cooling medium seal portion.

CONSTITUTION: A fluorescent screen panel 5 and a front-face panel 2 are respectively fixed by frame-like members 11 to which a high heat conductive bellows vessel 12 forming a variable capacity vessel is connected. This vessel 12 that is communicated with a cooling medium 6 filled between the panels 5, 2, has its simple structure requiring reduced number of parts so that the vessel 12 varies in capacity according to the expansion and contraction of the cooling medium for excellently absorbing the expansion and contraction while heat radiating property is heightened by the vessel having its heat conductivity.