Offered is a fluorescent substance consisting of Eu-activated β-sialon and capable of enhancing the brightness of a light emitting device such as a white LED using blue or ultraviolet light as a light source. The fluorescent substance has as its main constituent a β-sialon represented by the general formula Si6-zAlzOzN8-z and containing Eu, wherein the spin density is 2.0×1017/g or less as measured by electron spin resonance spectroscopy corresponding to an absorption of g=2.00±0.02 at 25° C. In the above fluorescent substance, it is preferable that lattice constant a of the β-sialon be 0.7608-0.7620 nm, the lattice constant c be 0.2908-0.2920 nm, and the Eu content be 0.1-3 mass %.