A method for preparing a sulfide phosphor where a raw material mixture for a sulfide phosphor is placed in an almost closed first heat-resistant container and fired therein, the method being characterized in that an auxiliary material composed of carbon and/or a carbon-forming compound capable of being pyrolyzed at a temperature not higher than the firing temperature to form carbon, and a metal sulfide capable of reacting with the carbon and/or said carbon-forming compound at a temperature not lower than 600°C but not higher than the firing temperature to form a carbon sulfide is placed in the portion as physically separated from the raw material mixture for the sulfide phosphor in the said first heat-resistant container and the firing is conducted under such condition so that said first heat-resistant container may have a carbon sulfide atmosphere by the thermal reaction of the thus fired auxiliary materials.