β-Sialon comprising Eu2+ that is present in a solid solution form in β-sialon represented by Si6-zAlzOzN μm [wherein z is 0.3-1.5], which shows, when excited with light of 450 nm in wavelength, a peak wavelength of fluorescent spectrum of 545-560 nm, a half-value breadth of 55 nm or greater, and an external quantum efficiency of 45% or greater. The β-sialon can be produced by blending at least one kind of oxide selected from aluminum oxide and silicon oxide with silicon nitride and aluminum nitride in such a manner as to give z of 0.3-1.5, further adding thereto a europium compound and a β-sialon powder having an average particle diameter of 5 μm or greater and an average degree of circularity of 0.7 or greater, each in a definite amount, and baking the mixture.