PURPOSE: To provide efficient adjustment by externally fitting a pre-saturated and -magnetized cylindrical magnet to an acceleration electrode, focussing an electron beam ahead of a phosphor screen, then demagnetizing a permanent magnet using a coil which is externally fit to a neck tube, and obtaining the optimum focus.

CONSTITUTION: A pre-saturated and -magnetized cylindrical magnet m is externally fit to an acceleration electrode g3 and an electron beam B is forcussed ahead of a phosphor screen. After other conditions such as acceleration voltage and the like are determined, the saturated and magnetized permanent magnet m is slowly demagntized and the electron beam is adjusted so as to be focussed just on the phosphor screen. Demagnetization is performed by applying AC or DC to coil C which is externally fit to neck tube N. As a result, a magnetic field lens is formed by externally fitting the cylindrical magnet to the acceleration electrode in the neck tube and its adjustment can be made efficiently.