PURPOSE: To prevent a phosphorous screen from ion burning effectively, by furnishing an ion beam screening plate between the electron gun and the phosphorous screen in order to pass only electron beams owing to a deflection through the side of the ion beam screening plate onto the effective surface of the phosphorous screen.

CONSTITUTION: The first to the fourth grids 12 to 15 accelerate and focus the electrons radiated from a cathode 11, but ions produced then around the cathode are accelerated and focused at the same time, and so electron beams including ions pass through around the center axis of the electron gun. In order to dispose of this passage, an ion beam screening plate 16 with an electron penetrating hole 161, eccentric to the center axis, is arranged at the outlet of the electron gun, and when no magnetic field is exerted, electron beams 21 including ions go straight on to hit the ion beam screen ing plate 16, but do not go out to the phosphorus screen 6. In this case, if a magnetic field 31 square to the center axis and horizontal is exerted in front of the screening plate 16, electrons which are of small mass are deflected and pass through the penetrat ing hole 161 to hit the phosphorous screen. On the other hand, ions which are of larger mass are not deflected broadly by the magnetic field, as that they go straight on to hit the screening plate 16, but not reach the phosphorous screen.