PURPOSE: To enable an electron gun to be concentrically sealed in the neck of a glass bulb by using a chuck having two vertically arranged right-side clicks and one left-side click.

CONSTITUTION: A chuck 19 has vertically arranged right-side clicks 20a and 20b which are brought into contact with the outer surface of the neck 1b of a blass bulb. Therefore, even when the clicks 20a and 20a are somewhat worn and torn, the slant of the glass bulb caused is much more smaller than that caused when a conventional device (having a right-side and a left-side click to be brougt into contact with the outer surface) is used because the two right-side clicks and one left-side click of the chuck 19 are pressed upon the outer surface. By using a device for sealing an electron gun in a cathode-ray tube in which the chuck 19 is used, the positional relationship between the glass bulb 1 and the electron gun 2 can be maintained correctly for a considerably long time. Therefore, use of this device enables a cathode-ray tube with very high quality to be manufactured.