PURPOSE: To restrict a color filter on both sides and improve the color purity of a projection tube by inserting the color filter in cooling liquid.

CONSTITUTION: A color filter 8 is installed in cooling liquid 7 in a liquid cooling tank 5 provided in contact with a front panel 2 of a projection tube 1. When the index of refraction of the cooling liquid 7 and that of the color filter 8 are close values to each other, reflection can be restricted on both sides of the color filter 8. When the pigment type color filter 8 is used, reflective light which may occur in a conventional multi-color filter is not produced for light beyond a necessary wavelength zone, and a good absorption property is achieved. Only light within the necessary wavelength zone can thus be taken out effectively, and the color purity of the projection tube 1 can be improved.