PURPOSE: To enable the main lens aperture in the same neck diameter to be expanded by arranging auxiliary electrode plates provided with an oval opening and a semioval notch part inside a cup electrode provided with a single ellipse- shaped opening part.

CONSTITUTION: The sides of the opposed surface of the G3 and G4 have single ellipse-shaped opening holes 311 and 411 of the completely same size each other having the respective edges of bent height H and inwardly bent-in horizontal size (h) and vertical size (v). Inside cup electrodes 31 and 41, auxiliary electron plates 33 and 43 are assembled by welding vertically to the tube axial direction in the positions retired respectively from the opposing end-faces by the fixed distances d1 and d2 each other. The auxiliary electrode plate 33 has a vertically long oval opening hole 331 to be the central electron beam passage out of the three electron beam passages and vertically long notch-shaped semioval openings 332 surrounding about half of the both side beam passages while the center of the semioval opening 332 accords with size S. The auxiliary electrode plate 43 has also the completely same constitution.