A method for preparing a phosphor having a host crystal comprising an oxide, which comprise steps of weighing, mixing and filling a powder of the phosphor, and thereafter one or more steps of firing the phosphor powder in a reducing atmosphere and, after the last firing in a reducing atmosphere, the step of firing the resultant phosphor in an oxidizing atmosphere, wherein the temperature for the step of firing in an oxidizing atmosphere is preferably 600 to 1000°C; and a plasma display device using a phosphor prepared by the method. A phosphor prepared by the method is reduced in the oxygen vacancy.