PURPOSE: To remove a needless electron generation source with good efficiency by impressing a magnetic field on a cathode-ray tube externally for making needless electrons to collide with a high-voltage electrode adjacent to the needless electron generation source.

CONSTITUTION: Since field emission 33 emitted from a projection part 32 of a peripheral part of a hole 28 of a second grid 15 is deflected in the peripheral direction by a powerful magnetic field 34 to be impressed from outside to collide with an electrode part near a hole 29 of a third grid 14, the colliding point part is heated. At this time, the third grid 14 is melted to emit a large amount of metal gas. The emitted gas bombards field emission 33 accelerated to high speed to be separated into ions and electrons for being accelerated by a high electric field impressed between electrodes so as to make needless electrons 33 to again bombard the electrode for emitting more gas so that a process such as activation of BaO at the tip of a projection part 32 and high-voltage treatment may be repeated. Discharge is generated near the projection part 32 by repeating such a phenomen to consume big energy and to smooth the projection part.