Alpha-siaion powder and method for producing the same

An +--sialon which is represented by general formula: (M1)x (M2) Y (Si, Al) 12 (O, N) 16 (wherein M1 represents one or more elements selected from the group which consists of Li, Mg, Ca, Y and lanthanoid (except La and Ce) and M2 represents one or more elements selected from the group which consists of Ce, Pr, Eu, Tb, Yb and Er and wherein 0.3 < X + Y < 1.5 and 0 < Y < 0.7) is synthesized by loading a container with a mixed powdery material of silicon nitride, aluminum nitride, an M1 containing compound and an M2 containing compound and at need aluminum oxide so that its bulk density is not more than 1.5 g/cm 3 , heat-treating the mixed powdery material at 1,600 to 2,000 DEG C in a nitrogen atmosphere. The +--sialon is pulverized to make an +--sialon powder, which can be utilized as a phosphor material for a while LED which uses a blue or an ultraviolet LED as its light source.