PURPOSE: To improve the working environment by maintaining a sufficiently stabilized high brightness even in photosensitive slurry or aqueous solution of water glass and reducing the amount of hydrogen sulfide gas generated in each process of coating, drying, film formation etc. in the phosphor-film formation process.

CONSTITUTION: Phosphorus compounds are added into the water, aqueous solution, or suspension which is used in the phosphor-film formation process by alkaline-earth metal sulfide phosphor material. According to the above described method the surface of phosphor particle made of alkaline-earth metal sulfide is coated with phosphorus compounds and stabilized for water. Thus, the hydrolysis tendency of the alkaline-earth metal sulfide phosphor material is suppressed by adding phosphorus compounds into he water used in the phosphor film formation process, and the trouble of formation of noxious hydrogen sulfide is solved. Further, as the chemical reactions between phosphor material and the photosensitive material bonding agents etc. which are contained in water are suppressed, a phosphor film having a high brightness can be obtained.