PURPOSE: To enhance the luminance and luminous efficiency of an electrofield luminescent lamp by allowing a fluorescent medium which is formed by sintering a mixture of in which Cu and Cl are added in the ZnS mother body.

CONSTITUTION: A mixture in which Cu and Cl are added in a ZnS mother body is first sintered so as to obtain an intermediate fluorescent medium 13b' consisting of particles having a hexagonal system type crystal structure. This first sintering is carried out for 1 to 8 hours at a temperature of 1,000 to 2,000 deg.C. Second, the intermediate luminescent medium 13' are agitated in a ball mill so as to deform the particles in order to cause crystal defect thereof. Then, the intermediate medium 13b' in which the crystal defect is caused, is second sintered so as to cause crystal dislocation a part of thereof from the hexagonal system type into the cubic system type, and then Cu is segregated between both types of crystals. The second sintering is carried out for 30 minutes to three hours at a temperature of 500 to 800 deg.C. After HCl etching is performed, it is dried so as to obtain a fluorescent medium 13b.