The present invention is to provide a phosphor of minimum particle size having an excellent crystallinity and a high brightness higher than conventional crude particles, which is usable for a highly precise display, a highly bright illuminating element or illuminating apparatus, a high speed immunoassay system or the like, i.e. a phosphor having a substantially spherical outer shape, which comprises primary particles having a median diameter D₅₀ in a range of from 0.05 m to 1 m and secondary particles having a median diameter D₅₀ in a range of from 0.1 m to 2 m, wherein at least 50 vol % of the total secondary particles has an aspect ratio of at least 0.8 and an internal quantum efficiency is in a range of from 0.8 to 1.