PURPOSE: To simply and accurately adjust color purity by arranging a magnetic body in which a color purity magnetic pole is formed at the opposed position inside a deflector and magnetizing it so that color purity can be obtained at the center of a fluorescent screen.

CONSTITUTION: Annular magnetic bodies 281 and 282 are mounted at the upper part of a convergence cup 268 and at the side lower part of the sixth grid 267, respectively. The convergence cup 268 is provided in the position where a deflector 29 is arranged. For example, when two poles are magnetized by a magnetizing device with removal of the magnetic bodies 281 and 282 and the deflector 29, the electron beam from an electron gun is defocussed and the brightness on the fluorescent screen which is formed on the inner side of a panel becomes dark and broad. As a result, color purity characteristics can simply be detected.