Abstract: A phosphor capable of exhibiting luminescence with high efficiency by excitation due to impingement of electrons accelerated under an anode voltage of hundreds volts to about 2 kV thereon and a method for preparing the same are disclosed. Mn is ion-implanted in ZnGa.sub.2 O.sub.4 which is a matrix crystal of the phosphor deposited on a substrate. Three kinds of implantation energy of 150 keV, 90 keV and 30 keV are selected and the ion implantation is effected three times in the order of the level of the implantation energy. A concentration of Mn is rendered substantially constant within a depth range between 100 angstrom and 1000 angstrom of a depth of 2000 angstrom or less in a luminous region of the phosphor. The ion implantation thus carried out three times permits the phosphor to exhibit increased luminous efficiency as compared with one-time ion implantation.