PURPOSE: To decrease the number of parts and production manhours, simplify the structure, and reduce the cost and weight by providing an electron gun corresponding to three phosphor layers in a vacuum envelope.

CONSTITUTION: An electron gun 4 is provided corresponding to three phosphor layers 2R, 2G, 2B in a vacuum envelope 1, and the first grid 17 and the second grids 18R, 18G, 18B for color selection are provided between individual phosphor layers 2R, 2G, 2B and the electron gun 4. Individual grids are applied with negative potentials in relation to a cathode 7 respectively, and a predetermined current is fed to a heater 6 to heat the cathode 7. Then, radiation quantities of non-focus electron beams 10RW10B radiated to phosphor layers 2RW2B through individual grids 17, 18RW18B are varied by individually changing potentials applied to the second grids 18RW18B after making the potential of the first grid 17 near the potential of the cathode 7.