PURPOSE: To obtain a fluorescent substance with a high efficiency of emission and improved chemical stability, by mixing an oxide of Mg, Ca, Sr, Ba, or Zn with a raw material of europium-activated yttrium oxide fluorescent substance and calcining the mixt.

CONSTITUTION: In the preparation of a fluorescent substance of the formula (wherein 0.01≤x≤0.20), a raw material of said substance and an oxide of one or more elements selected from Mg, Ca, Sr, Ba, and Zn and/or a compd. capable of forming an oxide of said elements by calcination are mixed and calcined. The amt. of said additives is 0.01W10mol%, pref. 0.5W3mol%. Calcination is carried out at 1,100W1,700°C, pref. 1,200W1,500°C, under an oxygen-bearing atmosphere.