The disclosed method can improve the luminescence life of an image display device. By using a blue luminescent phosphor having the phosphor component ZnS:M, N (where M is one or more elements selected from Cu, Ag, and Au, and N is one or more elements selected from Al, Ga, In, and Cl); wherein a shoulder is observed at around 400 nm (3.10 eV) of a cathode-luminescence spectrum of a field-emission display. A phosphor of a field-emission display of the present invention has improved characteristics against deterioration by electron-beam irradiation and an excellent luminescence maintenance factor, therefore, the luminescence life characteristics of a display are improved.