a one - gun three - beam electron gun comprises a triode part controlling an electron source generating a plurality of electron beams and emission of the electron beams generated by the electron source ; a main electron lens of bi - potential focusing type consisting of not less than two cylindrical electrodes focusing the plurality of electron beams emitted by the triode part ; electrostatic deflection plates disposed on the screen side of the main electron lens ; and electrostatic deflection plates disposed on the electron source side of the main electron lens .