PURPOSE: To prevent interference of the convergence magnetic field among each beam by providing a cylindrical screening body which screens horizontal and vertical deflection magnetic fiels around the central electron beam.

CONSTITUTION: A screening body 4 consisting of a cylindrical section and a T- shaped section which is stretched in the vertical direction is provided around the central electron beam. In this case, the vertical and horizontal magnetic fields are collected on the screening plate. The deflection sensitivity of both side beams B and R is reduced and the deflection magnetic flux is uniformly actuated for three electron beams. In this condition the convergence magnetic field is actuated by convergence devices 5 and 6. As a result, the interference of the convergence magnetic field can be prevented and convergence easily be adjusted.